Add folder-path signal to community similarity matrix

[raphasouthall]

Problem

Embeddings treat "infrastructure" as one topic whether a note lives under work/ or home/, so distinct organisational areas collapsed into one community. The largest coarse cluster was a work+literature+home soup (258 notes: work 203, literature 26, home 16).

Change

Blend a fourth channel into _build_similarity_matrix: notes sharing a top-level folder prefix get a uniform similarity bump.

• PATH_SIGNAL_WEIGHT = 0.3 (0 disables)
• PATH_PREFIX_DEPTH = 1 (top-level: work/, home/, research/…)
• S = 0.6·semantic + 0.25·cooc + 0.15·links + 0.3·path

Measurement (read-only sweep, ~490-note vault)

Purity = fraction of each community's notes from its dominant top-level folder.

config	Qc	nc	purity_c	Qf	nf	purity_f
baseline (δ=0)	0.308	9	0.68	0.257	13	0.76
depth1 δ=0.3	0.340	7	0.85	0.289	9	0.93
depth1 δ=0.5	0.424	5	0.94	0.317	9	0.94
depth2 δ=0.2	0.341	10	0.65	0.268	10	0.78

• depth=1 is the work/home grain; depth=2 reduced purity (splitting home/ weakens reinforcement).
• δ=0.3 lifts purity sharply while modularity holds/improves and counts stay sensible. δ≥0.5 over-purifies (clusters by folder, not topic) and collapses the count.
• Largest coarse cluster goes from work:203, literature:26, home:16 → work:211, literature:8, home:0.

Safety

Root-level files (CLAUDE.md etc.) form no path edges; a flat single-folder vault yields an all-zero S_path; weight 0 disables. So it degrades gracefully as a default.

Tests

New TestPathSignal: same-folder bump, cross-folder isolation, root-file exclusion, disable path. Full suite: 566 passed.