fix(backfill): chunk mv_replay from the MV source so empty targets bootstrap (#187)

[alvarogar4]

Summary

Fixes #187. A mv_replay backfill of a from-scratch empty aggregate target failed with No partitions found for <target>. The table may be empty. Chunk planning introspected the target's system.parts to size chunks and empty-checked it — but for mv_replay the target is legitimately empty (you're bootstrapping it), and the data actually read/sized is the view's source (its FROM table), not the target.

The fix detects the execution strategy before chunk planning and, for mv_replay with a single shared source, runs generateChunkPlan against that source table instead of the target:

• chunking/sql.ts — extractSourceTableRef() parses the first top-level FROM table out of a view's SELECT (handles db.table, bare table, backticks; rejects subqueries/table functions), reusing the existing quote/paren-aware scanner.
• detect.ts — resolveMvReplaySource() collapses the MVs feeding a target to a single shared source (unqualified FROM defaults to the view's database). Returns undefined — falling back to target introspection — when a source is unparseable or the views fan in from different sources, preserving the multi-source replay behaviour from fix(backfill): replay every MV feeding the target, not just the first (NUM-7480) #185.
• planner.ts — buildBackfillPlan now detects the strategy first, chunks the resolved source for single-source mv_replay, and the empty-check guards the introspected table (an empty source still fails fast). The copy path is unchanged.

Behaviour preserved

• copy backfills: unchanged (introspect the target == source).
• Multi-view fan-in from different sources (fix(backfill): replay every MV feeding the target, not just the first (NUM-7480) #185): unchanged (falls back to target introspection).

Test plan

• Unit: extractSourceTableRef (qualified/bare/backtick/alias/subquery/table-function/string-literal FROM), resolveMvReplaySource (single, shared, unqualified, differing sources, subquery).
• Unit regression: an empty target with a single-source MV no longer throws (source introspected); an empty source still throws with the source name.
• E2E (mv-replay-plan.e2e.test.ts, live ObsessionDB): populated source + empty SummingMergeTree target + MV → buildBackfillPlan plans one chunk per source partition, executeBackfill populates the target, per-bucket values match the forward MV output.
• typecheck + lint pass; existing planner/detect/sql/copy-path e2e tests green.

Note: pre-existing chunking/e2e/smart-chunking.e2e.test.ts failures in this environment are unrelated — they depend on a separately-seeded chkit_e2e_chunking_* dataset (UNKNOWN_TABLE) and fail identically with these changes stashed.

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https://claude.ai/code/session_013tzRKHhZETTZtQtDe999Fi