feat(blocks): block-vav effort medium→low (empirical calibration #208)

Set block-vav (בלוק ו', רקע/background) generation effort from medium to
low in BLOCK_CONFIG. The #208 empirical effort calibration
(scripts/calibrate_effort.py, report
data/eval/effort-calibration-20260630T155054Z.md, n=7 finals) found that
for block-vav, low effort lands closest to chair Dafna Tamir's finalized
text — winning on BOTH the composite distance metric (low 0.4454 vs
medium 0.4749 vs high 0.5381 vs xhigh 0.5413) AND the cleaner style
signal anti_pattern_total (low 1.57 vs medium 2.29 vs high 4.14). Chair
(chaim) approved adopting low for block-vav.

This is the only block with a clean/confident calibration signal —
block-zayin and block-yod-alef signals were ambiguous/tied and are left
unchanged. No other block effort is touched.

Invariants: G8 (effort empirically calibrated, not by-feel);
claude_session-local-only (block_writer runs via the local MCP/claude_local
generation path). Host-side generation parameter — after merge the host
tree needs `git pull` to take effect.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

mcp-server/src/legal_mcp/services/block_writer.py

@@ -65,7 +65,7 @@ BLOCK_CONFIG = {
     "block-gimel":{"index": 3, "title": "צדדים", "gen_type": "template-fill", "model": "script"},
     "block-dalet":{"index": 4, "title": "החלטה", "gen_type": "template-fill", "model": "script"},
     "block-he":   {"index": 5, "title": "פתיחה", "gen_type": "paraphrase", "model": "ai", "effort": "medium", "max_tokens": 4096},
-    "block-vav":  {"index": 6, "title": "רקע עובדתי", "gen_type": "reproduction", "model": "ai", "effort": "medium", "max_tokens": 16384},
+    "block-vav":  {"index": 6, "title": "רקע עובדתי", "gen_type": "reproduction", "model": "ai", "effort": "low", "max_tokens": 16384},
     "block-zayin":{"index": 7, "title": "טענות הצדדים", "gen_type": "paraphrase", "model": "ai", "effort": "high", "max_tokens": 16384},
     "block-chet": {"index": 8, "title": "הליכים", "gen_type": "reproduction", "model": "ai", "effort": "medium", "max_tokens": 8192},
     "block-tet":  {"index": 9, "title": "תכניות חלות", "gen_type": "guided-synthesis", "model": "ai", "effort": "high", "max_tokens": 16384},