build: introduce .context marker for distclean after partial builds

[Zepp-Hanzj]

Summary

Directory.mk's distclean target only traverses subdirectories that have .built, .depend, or .kconfig markers. When a build fails between the context:: and depend:: phases (e.g. during configure), none of these markers exist, so distclean silently skips the directory — leaving behind unpacked sources, downloaded archives, and generated config files.

This PR fixes this by introducing a .context marker and separating the scopes of clean and distclean.

Changes

1. Directory.mk — DISTCLEANSUBDIRS

Introduce a new variable DISTCLEANSUBDIRS: a superset of CLEANSUBDIRS that also includes directories carrying .context:

DISTCLEANSUBDIRS := $(CLEANSUBDIRS)
DISTCLEANSUBDIRS += $(dir $(wildcard */.context))
DISTCLEANSUBDIRS := $(sort $(DISTCLEANSUBDIRS))

• clean:: keeps using CLEANSUBDIRS — only clean compiled artifacts
• distclean:: now uses DISTCLEANSUBDIRS — also undo context:: work

2. Application.mk — clean .context on distclean

Add $(call DELFILE, .context) to the distclean:: target alongside .built, .depend, and Make.dep.

3. interpreters/python/Makefile — touch .context

Touch .context at the end of the context:: target, after unpacking CPython source. This ensures distclean can find the directory even if the build fails during configure.

4. interpreters/python/.gitignore

Add .context to the gitignore.

How it works

Scenario	.built	.depend	.context	CLEANSUBDIRS	DISTCLEANSUBDIRS	distclean cleans?
Successful build	✅	✅	✅	✅	✅	✅
Failed at configure	❌	❌	✅	❌	✅	✅ (new!)
Never built	❌	❌	❌	❌	❌	N/A

Verification

Simulated a partial build (context:: run, build fails at configure):

1. Created Python/, build/, install/, v3.13.0.zip, config.site, Setup.local
2. Created .context marker
3. Confirmed CLEANSUBDIRS is empty (no .built/.depend/.kconfig)
4. Confirmed DISTCLEANSUBDIRS includes python/ (has .context)
5. Ran distclean — all artifacts cleaned correctly

Fixes

Fixes #2895