fix(redact): narrow control-split join guard to line-crossing spans

[kshitijk4poor]

Summary

Narrows the control-split join guard added in #80965 (aecb9ca) so a token split by ESC/zero-width bytes is fully masked again — the guard as merged skipped the join whenever any fragment self-matched _PREFIX_RE, which left the non-matching tail of such a split in cleartext.

Root cause

The guard existed to stop a real bug (a complete token at end-of-line joining with the next text line and the mask swallowing "button [ref=e3]"). But its condition was broader than the bug: it also fired for non-newline splits where the head fragment alone was long enough to self-match, so sk-<15 chars>\x1b<25 chars> masked only the head and leaked the 25-char tail. (Upstream main never masked this shape at all — the merged state was still strictly >= main — but the salvage's own split coverage regressed.)

Fix

Skip the join only when the candidate span crosses a line boundary (\n/\r) — the only shape where adjacent legitimate text can be swallowed. ESC/zero-width controls never legitimately separate a token from prose, so joining there is safe.

Validation

Probe	Before (merged main)	After
sk-<head≥10>\x1b<tail>	tail LEAKED	head+tail masked
ghp_<token>\nbutton [ref=e3] annotation	preserved	preserved (unchanged)
ESC/ZWSP/NL splits, short fragments	masked	masked (unchanged)
two complete tokens sep. by ESC	both masked	both masked
env dump lines	intact	intact

• tests/agent/test_redact.py + tests/tools/test_browser_secret_exfil.py: 113 passed
• Both guard legs mutation-checked: reverting to unconditional skip fails the new tail-mask test; deleting the guard fails the annotation test
• ruff clean

Found by the post-merge final-stack review of my own follow-up commit on #80965. Refs #77484.