Fix: AddFiles() should respect commit.manifest-merge.enabled

[RSP22]

Summary

Closes #805

AddFiles() hardcodes .fastAppend() at line 780, bypassing appendSnapshotProducer()
which reads commit.manifest-merge.enabled. Setting this property has no effect when
using AddFiles() — manifests accumulate one-per-commit indefinitely regardless of the
property value.

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Root Cause

appendSnapshotProducer() already exists at line 126 and correctly reads the property.
It is used by AddDataFiles() and Append() but was missed in AddFiles():

// transaction.go line 780 — BEFORE (hardcoded, ignores property):
updater := t.updateSnapshot(fs, snapshotProps, OpAppend).fastAppend()

// AFTER (same as AddDataFiles() and Append()):
updater := t.appendSnapshotProducer(fs, snapshotProps)

Method	Uses appendSnapshotProducer()
AddDataFiles()	✅ yes
Append()	✅ yes
AddFiles()	❌ hardcoded .fastAppend() — this bug

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Java Reference

BaseTable.newAppend() always returns MergeAppend. MANIFEST_MERGE_ENABLED_DEFAULT = true
in the Java implementation. The Go implementation should behave consistently with the
reference implementation.

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Test Gap

Three existing tests set commit.manifest-merge.enabled=true but none call AddFiles():

Test	File	Uses AddFiles()?
TestCommitV3RowLineageDeltaIncludesExistingRows	snapshot_producers_test.go	❌ calls newMergeAppendFilesProducer() directly
TestMergeManifests	table_test.go	❌ uses AppendTable() → Append()
TestSetProperties	transaction_test.go	❌ only sets the property, never appends

The new regression test table/addfiles_merge_regression_test.go exercises AddFiles()
directly with commit.manifest-merge.enabled=true and verifies the merge fires.
It can be moved into transaction_test.go if preferred by reviewers.

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Changes

• table/transaction.go — 1-line fix: use appendSnapshotProducer() instead of hardcoded .fastAppend()
• table/addfiles_merge_regression_test.go — regression test proving the bug (fails before fix, passes after)

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Verification

Regression test:

• 3 sequential AddFiles() commits with minCountToMerge=2
• Before fix: 3 manifests (merge never fires)
• After fix: 1 merged manifest (merge fires on commit 3)

Production verification on a table with 1 commit/min:

• Before fix: HEAD snapshot accumulated 1121 manifests after 18 hours (one per commit, never merged)
• After fix: HEAD snapshot has 16 manifests after the next commit (merge fired)

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Note on AI Assistance

This PR was developed with AI assistance (Claude). The author has verified the
implementation, Java reference behavior, test coverage gap, and production impact end-to-end.

[RSP22]

Added table/addfiles_merge_regression_test.go as a standalone file rather than
adding to transaction_test.go for the following reasons:

1. Three existing tests already set commit.manifest-merge.enabled=true but none
   call AddFiles() — the gap is specific to AddFiles() and a separate file makes
   that immediately visible to anyone searching for test coverage.

2. Regression tests are easier to find when named after the bug —
   addfiles_merge_regression_test.go signals its purpose: prevent this specific
   regression from recurring. Contributors can locate it without reading
   transaction_test.go in full.

Happy to move it into transaction_test.go if preferred — the test itself
is unchanged either way.