Dispatch recipients on bech32 HRP instead of string prefixes

[pscheid92]

Summary

ParseRecipientLine (and its duplicate in the CLI) matched age1pq as a string prefix to detect ML-KEM recipients. But p and q are bech32 data characters, so roughly 1 in 1024 X25519 recipients legitimately starts with age1pq and was misrouted to the ML-KEM parser, failing with expected HRP 'age1pq', got 'age'. This is what broke the ParseRecipientsFile_MixedWithPlugin ubuntu leg on main (run 30005527741) and explains the sporadic local test failures seen during earlier work — the test generates a fresh random key each run. Plugins whose name starts with pq (age1pqfoo1...) were misrouted the same way.

Fix

• Dispatch on the true HRP: split at the last 1 (BIP-173 — data never contains 1), mirroring Bech32.Decode. age → X25519, age1pq → ML-KEM, age1<name> → plugin.
• The CLI's copied dispatch now delegates to AgeKeygen.ParseRecipientLine (made internal + InternalsVisibleTo("Age.Cli")), so the two sites can't drift again.
• Unparseable recipients now report unrecognized recipient: ... from both paths (previously two different messages).

A plugin literally named pq shares the ML-KEM HRP and remains resolved as native ML-KEM, same precedence as before.

Verification

• New deterministic regression tests around a brute-forced real X25519 keypair whose recipient begins age1pq (parses as X25519, round-trips encrypt/decrypt) plus a pqtest-named plugin recipient. The vector test reproduces the exact CI failure on the old code.
• Full suite: 363 unit tests + 143 CCTV vectors pass, zero build warnings.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.99%. Comparing base (97fe01a) to head (bb51c89).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #16      +/-   ##
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+ Coverage   91.97%   91.99%   +0.01%     
==========================================
  Files          41       41              
  Lines        2381     2385       +4     
  Branches      312      312              
==========================================
+ Hits         2190     2194       +4     
  Misses        133      133              
  Partials       58       58              

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