#813 rtx6000fix

Description

`--ple-io ram` refused to start on Windows (`no mlock; use --ple-io mmap`).
This wires the existing `strata::platform::lock_resident` helper
(working-set minimum + `VirtualLock`, chunked per GiB) into `PleTable::open`,
so the whole n-gram table is locked in RAM at startup on Windows too.
A lock failure still falls back to touching the pages (plain mmap
behaviour) with a warning, same as the POSIX path.

Changes
- `src/kernels/ngram.cpp`: Windows branch of the Mmap+lock path calls
  `lock_resident`; sets `locked()` on success.
- `src/program/generate.cpp`: drop the Windows `--ple-io ram` refusal;
  usage text now says VirtualLock on Windows, mlock elsewhere.
- `include/strata/kernels/ngram.hpp`: doc comments updated (28.8 GB
  IQ4_NL / 51.2 GB FP8, Windows semantics of `locked()`).

Measured on Windows, RTX PRO 6000 Blackwell 96 GB, 128 GB RAM, MSVC 19.44
- `platform_memory_test`: `locked 256 MiB via working-set minimum +
  VirtualLock`, OK.
- Full server start, 320,001,536-row IQ4_NL table (28.8 GB):
  `PLE table locked in RAM (--ple-io ram) in 95.9 s`, no fallback warning.