Pgtk::Stash caches non-deterministic queries (CURRENT_TIMESTAMP, RANDOM, etc.)

Description

Pgtk::Stash#select skips caching only for queries containing NOW() (with spaces):

cache(pure, key, params, result, ret, tables, marks) unless pure.include?(" NOW() ")

This misses many other non-deterministic SQL functions, which get cached with incorrect (stale) results:

Expression	Cached?	Correct?
NOW() (with spaces)	No	Yes
CURRENT_TIMESTAMP	Yes	No — bug
CURRENT_DATE	Yes	No — bug
CURRENT_TIME	Yes	No — bug
LOCALTIMESTAMP	Yes	No — bug
RANDOM()	Yes	No — bug
GEN_RANDOM_UUID()	Yes	No — bug
CLOCK_TIMESTAMP()	Yes	No — bug
NOW() (no spaces: NOW())	Yes	No — bug

The check is also whitespace-sensitive: SELECT NOW() is skipped, but SELECT NOW() with exactly no space before the paren passes through.

Impact

• Queries using any non-deterministic function besides NOW() return stale cached data
• Example: SELECT CURRENT_TIMESTAMP returns the same timestamp on every call
• Example: SELECT RANDOM() FROM generate_series(1,10) returns the same values every call
• The cache silently serves incorrect data with no indication of staleness

Suggested fix

Replace the single NOW() check with a regex covering the common non-deterministic expressions:

NONDETERMINISTIC = /\b(NOW|CURRENT_TIMESTAMP|CURRENT_DATE|CURRENT_TIME|LOCALTIMESTAMP|LOCALTIME|RANDOM|GEN_RANDOM_UUID|CLOCK_TIMESTAMP|TIMEZONE)\s*\(/i

def select(pure, params, result)
  # ...
  cache(pure, key, params, result, ret, tables, marks) unless pure.match?(NONDETERMINISTIC)
  # ...
end

Related previous work

This is a continuation of the same caching-correctness theme addressed in earlier issues:

• #368 — Stash table_mod non-monotonic stamp
• #364 — Stash skips invalidation for tables with underscores/digits
• #336 — Stash persistently-stale empty SELECT

Unlike those issues (which were about cache invalidation), this one is about cache prevention — queries that should never be cached in the first place because they are non-deterministic by nature.