Fix docs/API drift: descriptor_data and parse_characteristics batch flow

[RonanB96]

Library gap

Documentation and types suggest descriptor bytes can feed batch parsing, but the public API does not accept descriptor_data and nothing populates CharacteristicContext.descriptors.

Evidence (citations)

Docs claim to_parse_inputs produces input for parse_characteristics(..., descriptor_data=...):

• docs/source/how-to/usage.md lines ~284–321 — builds descriptor_data then calls parse_characteristics(char_data) without passing it
• src/bluetooth_sig/types/io.py — to_parse_inputs() docstring references parse_characteristics(char_data, descriptor_data=...)

Actual translator signature:

• src/bluetooth_sig/core/translator.py — parse_characteristics(char_data, ctx=None) only
• src/bluetooth_sig/core/parser.py — _build_parse_context() does not preserve ctx.descriptors

Architecture docs describe descriptor validation path that is not wired:

• docs/source/explanation/architecture/internals.md — “Descriptor Access Pattern”

Suggested approach (pick one direction — document decision in PR)

Option A (minimal): Fix docs to state descriptors are optional/manual; show read_descriptor() separately; remove false descriptor_data= API references.

Option B (small API): Add optional descriptor_data: dict[str, dict[str, bytes]] | None to batch parse, parse descriptors into CharacteristicContext.descriptors per char UUID — only if product owner wants validation enrichment (see issue #195 scope split).

This issue should at minimum eliminate doc/API contradiction. Implementation of full wiring can be a follow-up.

Acceptance criteria

• No doc examples reference non-existent descriptor_data= parameter unless implemented
• docs/source/how-to/migration.md checked for same drift
• If Option B chosen: tests prove Valid Range affects validation when descriptor bytes supplied
• Docs build / code-block tests pass if applicable

Key files

• docs/source/how-to/usage.md
• docs/source/how-to/migration.md
• src/bluetooth_sig/types/io.py
• src/bluetooth_sig/core/translator.py