descriptor: custom options, Any pack/unpack, and symbol-to-file lookup

[iainmcgin]

What

The final three reflection-parity items against protobuf-go / protobuf-es, so the answer to "what does Go's reflection do that buffa's doesn't" is only ever the documented deferrals (proto2 declared defaults, text format on dynamic messages, descriptor-tree navigation).

1. Custom options

Each linked descriptor (Field/Message/Enum/EnumValue/Service/Method/Oneof) exposes options() -> Option<&XxxOptions> — the raw generated options message, boxed and cloned at link time (None for the common no-options case). Matches Go's desc.Options().

Custom options are extensions of the *Options message and survive on its unknown fields. DynamicMessage::from_options(pool, field.options()?) re-decodes the options as a DynamicMessage of the corresponding google.protobuf.*Options type (resolved by MessageName::FULL_NAME), so dyn_opts.get(ext.field()) reads a custom option by name — no hardcoded type strings. Requires descriptor.proto plus the option-defining proto in the pool. This is the headline use case (protovalidate, (google.api.http) transcoding, PII annotations) and leans entirely on the extension machinery from #139.

2. Any pack/unpack

DynamicMessage::pack_any / unpack_any — the binary counterpart to the Any JSON @type expansion, for CEL dyn evaluation. unpack_any resolves the type_url's last path segment against the pool and decodes the value bytes; pack_any wraps a message with the type.googleapis.com/ prefix. AnyError (#[non_exhaustive], Clone) distinguishes NotAny / AnyNotRegistered / MissingTypeUrl / UnknownType / Decode { type_url, source }.

3. Symbol → file

DescriptorPool::file_containing_symbol is now backed by a precomputed symbol_file index — O(log n), replacing an O(symbols × files) parent-walk scan that only covered messages and enums. It indexes the full set gRPC server reflection's FindFileContainingSymbol accepts: messages, fields, oneofs, enums, enum values (in their parent scope per protobuf naming), services, methods, and extensions. Unblocks a ServerReflection adapter in connect-rust.

Verification

• 22 new e2e tests across the three features (custom options on field/message/method, Any pack/unpack round-trip + all error paths, symbol→file for every symbol kind).
• Reflect conformance suite holds at 2783 successes, 0 expected failures, 0 unexpected.
• Workspace clippy clean, reflect-only no_std clean, fmt clean, doctests pass.
• Both review agents' Critical/High/Medium findings addressed.

Net change

+~560/-36. Stacked on #139 (extension reflection) — the custom-option read path depends on it.

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