Follow up RIODescInfo consolidation: target info, size and IO capabilities

Follow-up to #26121, which consolidates device/debugger flags and listener detection in RIODescInfo.

The goal is to remove duplicate callbacks and query wrappers, simplify consumers, and keep descriptor information cheap to obtain. This is a candidate inventory for discussion, not a proposal to add every field.

listener is the clearest additional consolidation: it already had a separate plugin callback and r_io_is_listener() wrapper. It is being handled in #26121; the remaining options are tracked here.

The strongest candidates for consolidating existing queries:

Candidate	What it provides	Main consideration
listener	Whether this descriptor is accepting connections	Handled in #26121 by removing the separate callback and wrapper.
pid, tid	Process and thread associated with the descriptor	Define whether these mean attached or currently selected; existing plugins differ.
baddr	Main executable's load base	Distinguish it from the start of an IO mapping.
size	Length of the exposed data, including device capacity in bytes	Must distinguish unknown size from an empty descriptor and an address-space limit.
size_kind or size validity	Exact, unknown, or unbounded size	Could replace ambiguous size sentinels.
seek_caps	Supported positioning: absolute, relative, end, holes	More useful for IO decisions than using chrdev as a proxy for seek behavior.
caps	Read, write, resize, extend, accept capabilities	Describe this descriptor's capabilities. A callback existing does not guarantee the operation works.
Resource classification	File, block device, character device, stream, memory	Keep independent properties such as isdbg separate; a remote debugger can also expose memory.

Device geometry and transfer constraints are also eligible, provided something consumes them:

Candidate	What it enables	Main consideration
logical_block_size	Correct sector addressing and minimum device access units	Separate from total device capacity.
physical_block_size	Better alignment of device writes	Different from logical sector size.
Read/write offset alignment	Alignment of the requested address	Read and write requirements can differ.
Read/write length alignment	Valid transfer lengths	Separate from address alignment.
Buffer alignment	Requirements on the caller's memory buffer	Only expose restrictions the plugin passes through to callers.
min_io, preferred_io	Minimum and efficient transfer sizes	Required constraints and performance hints need distinct meanings.
max_read, max_write	Generic chunking for remote or device IO	Report usable payload sizes, not raw protocol packet sizes.
page_size	Target memory page granularity	Target page size can differ from the host's.
allocation_granularity	Alignment for allocation or mapping	Not necessarily the page size.
erase_block_size, write granularity	Flash programming constraints	Useful for device backends; probably a separate feature.
Address bounds	Lowest/highest address the descriptor can represent	Bounds do not establish that every address inside is readable.
addrbytes / address unit	Conversion between addresses and byte counts	Would require reconciling the existing RIO.addrbytes setting.
Address-space kind	File offsets, process virtual memory, physical memory, registers	Useful only if generic code makes a concrete decision from it.
Sparse/hole behavior	Whether gaps exist and how reads of gaps behave	Actual extents need a separate query or iterator.

Other plausible options, with weaker justification for the initial consolidation:

Candidate	Potential use	Why to be selective
volatile / externally mutable	Cache invalidation decisions for live memory or changing resources	Ordinary files can change externally too; define a precise guarantee.
Read/write side-effect flags	Avoid repeated reads of device registers or MMIO	Often belongs to individual ranges rather than the whole descriptor.
Growing / fixed-length / snapshot properties	Decide whether cached size remains useful	Distinct from whether callers may resize it.
Native FD or handle	Polling, OS integration, zero-copy operations	Needs a tagged, borrowed handle; it is different from r2's descriptor ID.
isatty	Terminal-specific behavior	Character device alone does not imply terminal.
Nonblocking / append mode	Explain effective IO semantics	Describe the exposed descriptor, including any buffering performed by the plugin.
Transport / remote status	Diagnostics and connection presentation	Prefer actual capabilities over decisions based solely on "remote".
Connection state, EOF, readiness, available bytes	Stream handling	These become stale immediately; explicit status/poll queries usually fit better.
Current offset	Replace queries that use seek-to-current	Existing global and plugin-local offsets need consistent semantics first.
Backing descriptor and offset	Describe slices and layered resources	Compression and transformations cannot always be represented as a simple offset.
File identity, timestamps, allocated size	Detect replacement/modification; describe sparse storage	This starts becoming a separate stat API.
Target executable path	Avoid textual debugger commands for executable discovery	String ownership and remote-query cost complicate a small value struct.
Architecture, bits, endian, OS hints	Initialize target configuration from a backend	Keep ownership with the appropriate architecture/debug layer; avoid conflicting copies.
Generation counter	Tell consumers when cached information changed	Only useful with a defined update/invalidation contract.

Avoid duplicating fd, permissions, name, URI, plugin identity, or configuration already accessible through RIODesc. Registers, stop reasons, thread/module lists, and memory maps belong in their existing debug/map APIs.

The limiting factor is query cost. r_io_desc_info() is called during ordinary reads and writes in libr/io/io_plugin.c. Querying sysgdb's PID sends a debugger command; querying descriptor size can perform three seeks. Those operations cannot become an automatic consequence of checking a flag.

The suggested priority after listener consolidation is PID/TID/base consolidation. Seek capabilities are the strongest additional behavioral improvement. Size and geometry deserve an explicit query-cost contract first. Optional values also need validity indicators: zero is a valid size or base address, and "unknown" must remain distinguishable.