feat(protocol): report taproot support in Features

[BitHighlander]

Adds a single field to Features:

optional bool supports_taproot = 27;

Why

keepkey-firmware sets this in fsm_msg_common.h so a host can detect P2TR
capability directly instead of inferring it from a firmware version string,
which breaks whenever the feature is retargeted to a different release.

Why this is separate from #112

#112 carries the same field, and this PR keeps field number 27 and the exact
wording so the two agree and do not conflict when #112 lands.

It is split out because #112 is the full 7.15 proto surface (hive, near, ripple,
solana, zcash — 3,071 lines). Pinning that into the 7.14.3 bitcoin-only release
branch pulls in structured EIP-712 and Solana LUT fields that have no .options
entries in that tree, so nanopb emits pb_callback_t for them and both the ARM
and emulator builds fail with:

pb_callback_t forbidden. missing .options entry?

7.14.3 needs supports_taproot and nothing else from this repo — no source file
on that branch references any other field #112 adds.

Checks

• protoc parses messages.proto clean
• field 27 is unused on master (Features has 1-18, 21-26)
• no duplicate field numbers after the change
• additive only: 3 lines, no field renumbered or removed