Tracking fws U2: canonical advisory lock key for file writes #2017

What this unit delivers: the advisory lock key is canonicalised through the symlink referent, so a symlink alias and the file it points to share one lock.

Why it matters: two writers that reach the same file through different paths (a symlink and the real path) currently take different locks and can overwrite each other. Every writer in the chain picks its lock through this key.

Implemented by: PR #1911 (branch fws/u2-lock-key).

Acceptance:

• The lock key resolves the target through its symlink referent before locking.
• An alias and its referent serialize on one lock; unrelated paths keep independent locks.

Part of the file-write-safety epic #1375, landed as a nine-PR chain tracked on #1991. Merge order: U1 -> U2 -> U3 -> U4 -> U5 -> U8 -> U6 -> U7 -> U9.