#2599 docs: the READY timeout (900 s, STRATA_ENGINE_READY_S) in DETAILS.md's table and TROUBLESHOOTING.md (#2331)

Description

The docs part of #2331 (#2575 names the setting in the error message itself).

**What it changes.** The server ends the engine when it has not reported READY within 900 s (`STRATA_ENGINE_READY_S`, `serve/server.py`; 0 or less means no limit). A load from a slow disk can need longer: on #2331's box (the 28.8 GB n-gram table from HDD-class storage with `--ple-io ram`: 454-508 s for that table alone) every UD-Q4_K_XL start ended at ~15 min, with the engine's log cut off mid-load and the reason only in the server's log. Now:
- `docs/DETAILS.md`'s troubleshooting table has a row for `the engine did not report READY within 900 s`, with those numbers and the setting;
- `docs/TROUBLESHOOTING.md`'s "The first start takes minutes" entry says the same in two lines (`STRATA_ENGINE_READY_S=3600`, the reporter's value, or 0).

**What it leaves alone.** The code and the 900 s default; every other line. No new measurement: the numbers are the reporter's.

**How it was checked.** Against `ENGINE_READY_S` / `_timeout_env` and the message in `serve/server.py`, and #2331's comment; a sub-agent reviewer checked both.

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