Enforce the semaphore count <= max_count invariant

[seanwevans]

Bug

create_semaphore accepted parameters that produced an impossible semaphore:

SELECT create_semaphore('broken', 5, 1);
SELECT name, count, max_count FROM semaphores WHERE name = 'broken';
--   name  | count | max_count
-- --------+-------+-----------
--  broken |     5 |         1   <- 5 held permits, max of 1

The table only checked count >= 0 and max_count >= 1, never their relationship, so a semaphore could be created already over its own ceiling.

Fix

• Add CHECK (count <= max_count) on the semaphores table (enforces the invariant even against direct inserts and the existing acquire/release paths, which already keep count within [0, max_count]).
• Validate the arguments in create_semaphore so callers get a clear message instead of a silently broken (or cryptic constraint-named) error.

Test

Adds semaphore_validation, covering the accepted case plus the three rejected ones (count > max, negative count, max < 1) and confirming the invalid semaphores are not created. Suite green (14/14 on this branch).

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