feat(pdo): expose statement rows as a lazy seq (statement-seq)

[Chemaclass]

Wraps the intent of PDOStatement::getIterator as pdo/statement-seq: a lazy seq of the statement's remaining rows as maps, fetched one row at a time.

(->> (pdo/query conn "select * from big")
     (pdo/statement-seq)
     (map :name)
     (take 10))   ;; only 10 rows are fetched

Return-shape decision (per #16)

Returning the raw PHP Iterator would force callers into php/->, and an eager vector would just duplicate fetch-all. Instead statement-seq is built on lazy-seq + the existing fetch, so:

• it is genuinely lazy - (first ...) / (take n ...) only fetch what they consume;

• rows are keywordised maps via row->map, consistent with fetch/fetch-all (and with a stable FETCH_ASSOC, unlike a raw iterator whose shape depends on the fetch mode).

• Removed getIterator from the "Not implemented yet" block in src/pdo/statement.phel.

• Tests: statement-seq-yields-rows-as-maps (full realisation) and statement-seq-is-lazy-first-only-fetches-head (first realises just the head).

• README API table + CHANGELOG ## [Unreleased] updated.

Polish pass: no refactor needed - composes lazy-seq + fetch + cons, no new state.

composer test green (49 assertions).

Closes #16