fix: materialize namespace packages concretely in venv site-packages

[jbedard]

See #1092 instead

Changes are visible to end-users: no

Test plan

• Covered by existing test cases
• New test cases added

[aspect-workflows]

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📅 Mon Jun 8 02:14:17 UTC 2026

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[github-actions]

py_binary startup benchmark

Version	Mean (ms)	Median (ms)	± stddev	vs BCR	vs main	Build (s)
BCR 1.11.5 (baseline)	324.299	322.768	±8.777	—	—	42.56
HEAD main	61.912	61.967	±0.894	-80.9%	—	11.91
This PR	61.686	61.477	±1.059	-81.0%	-0.4%	10.84

Measured with hyperfine --warmup 5 --runs 50 on Linux
Gate: PR vs HEAD main (threshold: 10%). BCR is shown only as a historical baseline.
Build time: cold bazel build //:bench with isolated output base, no disk cache.

sys.path quality

Version	sys.path entries	distinct site-packages roots	duplicate realpaths
BCR 1.11.5 (baseline)	6	1	0
HEAD main	7	2	0
This PR	7	2	0

sys.path quality measured by bench_syspath inside the assembled venv. Duplicate realpaths indicate symlink redundancy; many distinct site-packages roots suggest an inefficient venv layout.