The _hermes-python prologue compared uv.lock / pyproject.toml / pm/lock.json
against facts.json with `-nt`. facts.json is only rewritten on a real sync,
so a checkout that rewrites an input without changing it left the input
newer forever: every run from an activated shell re-sourced activate
(~370ms instead of ~13ms).
pm now records, after every successful full-closure install (no-op syncs
included), a stamp per input under installs/<key>/inputs carrying the exact
mtime the install was verified against, snapshotted before installing so
an input edited mid-install still reads as stale. The prologue re-activates
when any input's mtime differs from its stamp in either direction (branch
switches can move it backwards), when an input is missing, or when there
are no stamps. It globs the stamp dir, so pm owns the only input list.
Also read pyvenv.cfg as utf-8-sig (footgun lane, same file).
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