Default child environment: HOME is not forwarded and a PATH change re-runs nothing, neither pinned by a requirement #480

Symptom

putup's default child environment diverges from tup's default_env[] in two ways that no requirement in spec/requirements/ states, so a reader diffing against upstream finds them as defects rather than decisions.

1. HOME is not forwarded. tup's default_env[] (src/tup/environ.c) is PATH and HOME on POSIX, plus a Visual Studio set under _WIN32. putup's base_child_env() (src/platform/process-posix.cpp) gives PATH, and since Build commands get PATH only, so TMPDIR is dropped and GCC writes temporaries into the source tree when /tmp is read-only #478 TMPDIR/TMP/TEMP when set. A probe rule : |> echo "HOME=[$HOME]" > %o |> out.txt prints HOME=[] under putup. Tools that resolve per-user state from HOME (ccache is why tup forwards it, and tup also injects CCACHE_NODIRECT=1 in tup_db_get_environ) fall back to getpwuid or fail; which tools break is not measured.

2. A PATH change re-runs nothing. tup makes every default-environment name a sticky env node of each command (environ_add_defaults, tup_db_check_env), so a changed PATH re-runs every command; the man page says so under export. putup forwards PATH without recording it. Reproduced on main 75c93ee with : |> echo "PATH=$PATH" > %o |> out.txt: build with PATH=/usr/bin:/bin, rebuild with PATH=/usr/bin:/bin:/opt/zzz, putup prints Nothing to do (up to date) and out.txt keeps the first value. The only route by which PATH reaches identity is CONFIG_TRACKED_TOOLS resolving a bare name through it.

Where this stands in the spec

#479 added REQ-ENV-TEMPDIR and REQ-ENV-TEMPDIR-IDENTITY for the temporary-directory variables and noted the HOME divergence in a reference: clause only. PATH's forwarded-not-recorded status has no requirement; docs/reference.md §export now states it in prose.

Decisions to make

• Whether to forward HOME (conform to tup) or declare its absence as a deviation with a requirement. If forwarded, whether it joins identity: the same per-session concern that kept TMPDIR out applies less (a user's HOME rarely changes between builds), but tup's behaviour of re-running on any default-variable change is what a tup-conformant label would have to mean.
• Whether PATH changes should re-run commands as in tup, or stay excluded with a requirement stating so and naming CONFIG_TRACKED_TOOLS as the deliberate route. Re-running on every PATH change is the tup behaviour the man page warns about ("if PATH is changed, all commands will run again").

Either way the outcome is a requirement per variable in command-record.ears.md's env-values group with a discharging scenario, so the divergence stops being silent.

Found by the #478 design and review passes.