feat: implement hash join

[RichardKnop]

Nested loop join (what minisql does today, with index on the join column):

• Complexity: O(N × log M) with an index, O(N × M) without
• Best for: small tables, OR when there's a useful index on the inner table's join column
• Memory: essentially O(1)

Hash join (equi-join only):

• Complexity: O(N + M) — linear
• Best for: large tables with no useful join-column index, where you'd otherwise pay O(N × M)
• Memory: O(min(N, M)) — must materialise the smaller ("build") table into a hash map

So hash join is faster than unindexed nested loop on large tables, but it costs memory. The 64MB threshold doesn't mean "switch back to nested loop because it's better there" — it means "beyond this size, the build-side hash table may not fit in RAM, so in-memory hash join is no longer safe to use." A production DB would do grace hash join (spill to disk) instead; for minisql it's reasonable to just fall back to nested loop.

Corrected strategy for minisql:

Condition	Plan
Join column has an index	Indexed nested loop (current behaviour) │
│ No index, build side ≤ threshold	In-memory hash join — O(N+M) │
│ No index, build side > threshold	Nested loop sequential — O(N×M), slow but no memory risk │

The threshold protects memory, not because nested loop is algorithmically better at large scale — it's strictly worse without an index. So the plan should be inverted: use hash join for large-enough tables where a full scan is needed (making it worth the memory cost), and keep nested loop for small tables or indexed joins.

[github-actions]

Code Coverage

✅ Total: 70.2% (threshold: 70%)

Package	Coverage
github.com/RichardKnop/minisql	80.7%
github.com/RichardKnop/minisql/e2e_tests	[no
github.com/RichardKnop/minisql/internal/minisql	69.5%
github.com/RichardKnop/minisql/internal/parser	84.2%
github.com/RichardKnop/minisql/pkg/bitwise	100.0%
github.com/RichardKnop/minisql/pkg/lrucache	81.7%