watchman: cache the socket path

[jstasiak]

When no socket is provided watchman_client runs full Watchman CLI solely to discover it and running Watchman is slow-ish (to the point where it can dominate the cost of short jj commands), at least in the environment I tested (macOS 26.5.2, M1 Pro).

50 runs test (small repo to make the difference more obvious, the actual watchman get-sockname command to demonstrate the point):

Command                   Mean ms   Min ms  Median ms   Max ms
========================  =======  =======  =========  =======
watchman get-sockname     133.173  129.363    132.186  161.242
jj status (0.44.0)        166.157  161.038    165.656  187.851
jj status (this patch)    36.437   32.206     34.483   96.386

Remember the last endpoint in the platform cache directory and validate it with a cheap protocol request before use. Fall back to normal CLI discovery and refresh atomically when it is stale. Explicit nonempty WATCHMAN_SOCK values retain their existing behavior and do not affect the cache.

serde_bser is not actually a new dependency, watchman_client already pulls it in.

Checklist

If applicable:

• I have updated CHANGELOG.md
• I have updated the documentation (README.md, docs/, demos/)
• I have updated the config schema (cli/src/config-schema.json)
• I have added/updated tests to cover my changes
• I fully understand the code that I am submitting (what it does,
  how it works, how it's organized), including any code drafted by an LLM.
• For any prose generated by an LLM, I have proof-read and copy-edited with
  an eye towards deleting anything that is irrelevant, clarifying anything
  that is confusing, and adding details that are relevant. This includes,
  for example, commit descriptions, PR descriptions, and code comments.