Fix RAND() function behavior

[JanJakes]

Summary

Replaces the old mt_rand(0, 1) stub — which returned an integer 0 or 1, not a float in [0, 1) as MySQL requires — with MySQL-compatible RAND() behavior.

Replaces #341.

Unseeded RAND()

Compiles to a native SQLite expression:

((RANDOM() & 0x001FFFFFFFFFFFFF) / 9007199254740992.0)

The 53-bit mask matches the IEEE 754 double mantissa, so division by 2^53 is exact and strictly less than 1.0. Matches MySQL, where unseeded RAND() uses a thread-level state independent of RAND(N).

Seeded RAND(N)

Routes through a PHP UDF implementing MySQL's exact LCG from my_rnd_init() / my_rnd() (sql/item_func.cc, mysys/my_rnd.cc), bit-exact against MySQL 9.6. Requires 64-bit PHP.

Seed handling matches val_int(): NULL becomes 0, floats round to nearest (RAND(3.9) == RAND(4)), numeric strings follow the same path.

Known divergences

• MySQL distinguishes constant vs non-constant seeds at parse time (constant = init once per statement, non-constant = reinit per row). A SQLite UDF can't see the expression, so we approximate by reinitializing only when the seed value changes. This diverges when a non-constant expression yields a stable value.
• The UDF keeps one LCG state per connection, so multiple RAND(N) call sites in one query share a stream: SELECT RAND(1), RAND(1) returns (v1, v2) here vs (v, v) in MySQL.

Both are documented in the rand() docblock.

Metadata

RAND() column metadata is now reported as DOUBLE / PARAM_STR, removing a long-standing TODO.

Test plan

• CI green.
• Bit-exact reference values against MySQL 9.6 (seeds 0, 1, 3, 5, and multi-row sequences).
• NULL, float, numeric/non-numeric string, and negative seed handling.
• RAND() in WHERE, UPDATE, INSERT, ORDER BY (LIMIT + seeded deterministic permutation).
• Per-statement flush contract inside a transaction.