feat(c): add write/commit C FFI bindings

[suxiaogang223]

Summary

Add write-side C FFI bindings (bindings/c/src/write.rs) following the same opaque handle / extern "C" patterns as the existing read-side bindings. Closes #520.

API surface (14 functions)

// WriteBuilder
paimon_table_new_write_builder(table)        → paimon_result_write_builder
paimon_write_builder_free(wb)                → void
paimon_write_builder_with_overwrite(wb)      → *mut paimon_error
paimon_write_builder_new_write(wb)           → paimon_result_table_write
paimon_write_builder_new_commit(wb)          → paimon_result_table_commit

// TableWrite
paimon_table_write_free(tw)                  → void
paimon_table_write_write_arrow_batch(tw, array, schema) → *mut paimon_error
paimon_table_write_prepare_commit(tw)        → paimon_result_prepare_commit

// TableCommit
paimon_table_commit_free(tc)                 → void
paimon_table_commit_commit(tc, msgs)         → *mut paimon_error
paimon_table_commit_overwrite(tc, msgs)      → *mut paimon_error
paimon_table_commit_truncate_table(tc)       → *mut paimon_error
paimon_table_commit_abort(tc, msgs)          → *mut paimon_error

// CommitMessages
paimon_commit_messages_free(msgs)            → void

Design decisions

• In-process transfer: CommitMessages are passed as opaque handles (no serialization), matching Python bindings
• Shared commit_user: WriteBuilder generates a UUID shared by writer and committer
• Arrow C Data Interface: Batch import via FFI_ArrowArray/FFI_ArrowSchema raw pointers

Tests (15 cases, all passing)

• Read path (3), Predicate (3), Write path (7), Error handling (1), Integration (1)

Run: cargo +1.91.0 test -p paimon-c

[suxiaogang223]

Thanks for the detailed review, @QuakeWang! I added regression tests to reproduce each of the issues you pointed out and confirmed the failures before making the fixes. The new tests now cover Arrow C Data ownership, invalid and schema-mismatched batches, commit message provenance and lifetime, index cleanup on abort, and caller-provided commit identity with idempotent retries. Everything is passing now, including the full paimon-c test suite and the targeted core tests. The fixes are in 27a9d90. When you get a chance, could you please take another look? Thanks!

[JingsongLi]

No issue from me.