#35918 descriptor: Expose key expression parsing

• Allow callers like the GUI multisig wizard to parse and inspect standalone key expressions
• Reject invalid trailing characters when parsing multisig expressions

This PR moves the abstract `PubkeyProvider` interface into `src/script/descriptor.h`, exposes a public `ParsePubkey()` overload to parse standalone key expressions, adds a `GetOriginInfo()` method to inspect key origin metadata, and tightens the `musig()` parser to reject invalid trailing characters.

Problem: Higher-level callers like the GUI multisig setup wizard need a way to parse, validate, and inspect standalone key expressions without constructing a full output descriptor.

P3 · new feature

• P3 because it unblocks GUI multisig setup flows without affecting core wallet safety
• Enables validating pasted key expressions and inspecting key origins
• Fixes improper acceptance of trailing characters after multisig expressions

P3 because it provides a useful utility to unblock GUI multisig setup flows (#35645) without affecting core wallet consensus or fund safety. The description explicitly frames this as useful for the GUI multisig wizard tracking issue (#35645) to validate pasted key expressions and inspect key origins. It also fixes a minor issue where trailing characters after musig() were improperly accepted.

Membership: Touches descriptor interfaces in src/script/descriptor.h and src/script/descriptor.cpp, which define descriptor parsing for the wallet.

Factors: security/stability 0, bug 1, performance 0, user value 1, leverage 2

• Needs rebase due to merge conflicts

The PR has merge conflicts and an open question from uqlidi regarding whether `ParsePubkey` should parameterize the script context instead of hardcoding P2TR has been unanswered for 19 days.

Author status: silent since 2026-08-11

Open concerns:

• uqlidi asked why the public `ParsePubkey` overload hardcodes `ParseScriptContext::P2TR` rather than taking the context as an argument.

• Support on the latest commit without objections (w0xlt)
• Questions whether to parameterize the script context rather than hardcode taproot (uqlidi)

Positive: ACKed by w0xlt, with an open API design question from uqlidi.

w0xlt provided an ACK on the latest commit. uqlidi raised a constructive API design question about supporting script contexts other than P2TR, but did not object to the concept.

• w0xlt: ACK 93600f9f03
• uqlidi: suggested parameterizing ParseScriptContext rather than hardcoding P2TR

Review verdicts (DrahtBot): 1

• ACK: w0xlt

Enables:

• #35645

185 lines under test/bench/ci.

• src/test/descriptor_tests.cpp +185/-0
• src/script/descriptor.cpp +32/-86
• src/script/descriptor.h +85/-0

PR #35918 exposes the `PubkeyProvider` interface and adds a public `ParsePubkey()` function to allow standalone descriptor key expression parsing, validation, and origin inspection. This utility is needed by UI workflows such as the multisig wizard (#35645) to validate user-pasted key expressions before creating descriptors. It also fixes a minor parsing bug to reject unexpected trailing characters in `musig()` expressions. The PR has an ACK from w0xlt but is currently dirty and waiting on an author response to an API question from uqlidi.