Problem: output comparison used exact string equality after whitespace
normalisation, causing correct solutions to fail on problems where
floating-point answers are accepted within a tolerance (e.g. 1e-6).
Solution: add an optional ui.panel.epsilon config value. When set,
actual and expected output are compared token-by-token: numeric tokens
are compared with math.abs(a - b) <= epsilon, non-numeric tokens fall
back to exact string equality. Per-problem epsilon can also be stored
in the cache and takes precedence over the global default.
Problem: M._cache = cache_data captured the initial empty table reference
at module load time. After M.load() reassigns cache_data to the decoded
JSON, M._cache is permanently stale and returns the wrong table.
Solution: remove the field assignment and expose get_raw_cache() which
closes over cache_data and always returns the current table.
Problem: after an install or update, the on-disk cache may contain data
written by an older version of the plugin whose format no longer matches
what the current code expects.
Solution: embed a CACHE_VERSION in every saved cache file. On load, if
the stored version is missing or differs from the current one, wipe the
cache and rewrite it. Corrupt (non-decodable) cache files are handled
the same way instead of only logging an error.
