Feature/heap

[theweird-kid]

Implements DD-004: the slotted heap page and the table heap built on top of it.

What's in it

• HeapPage — slotted page over the 4064-byte body. Slots never move; tuple bytes move freely,
  so compaction rewrites the tuple region without invalidating any RID. Dead-slot reuse,
  dead_bytes accounting, and CheckInvariants() after every mutation.
• TableHeap — a chain of HEAP pages linked through next_page_id. Insert/Get/Delete/Update/Scan.
  • Chain extension uses the last page's write latch as the extension lock; the link is re-read
    under the latch, so concurrent inserters never allocate over each other.
  • v1 free-space reuse: a rotating insert hint plus compact-on-probe, bounded at 4 probes before
    appending.
  • Update relocates when a row outgrows its page (insert first, then delete) and returns the new RID.
• TableIterator — forward scan that holds one guard at a time and copies each tuple out.
  Documented hazard: mutating through a live iterator self-deadlocks; use collect-then-mutate.
• BufferPoolManager fix — FetchFrame rejects INVALID_PAGE / META_PAGE_ID up front.
  Previously both took the miss path and aborted debug builds on an assert in AbandonLoad.

Tests

• heap_page_test.cpp and table_heap_test.cpp (32 tests, six tiers: durability across reopen,
  row lifecycle and error codes, chain and scan, space accounting and compaction, update, concurrency).
• Concurrency tests cover the chain-extension race and check the file itself:
  PageCount() == chain pages + reserved, so a leaked page fails the test.
• 144/144 ctest green under asan and tsan locally; concurrency tests 30/30 under tsan.