feat: field-bound @ SAY GET — promotes LOOKUP to full end-to-end (#59)

[DDecoene]

Summary

Makes @ SAY GET bind directly to a table column when one matches — the dBASE III behavior WebBase-III's memory-variable-only forms had deviated from until now.

• Executor.doAtSayGet recognizes a GET whose name matches the active table's column (case-insensitive) and binds to it, capturing the record's rowid and prefilling from its current value. Fields take precedence over a memory variable of the same name — the reason the m_ prefix convention exists. A field-bound GET requires a current record (** Error: GET <field>: no current record otherwise — this must be a hard failure, not a silent no-op).
• If the bound column has a LOOKUP, the form renders the same picker BROWSE does; an unresolvable lookup degrades to free entry with a warning, same as everywhere else.
• form-submit writes field-bound values by their captured rowid — never by trusting the client's message for which columns exist on the form, mirroring grid-edit's authoritative re-check. Submission is all-or-nothing: every field is validated (type + fresh lookup re-resolution) before any is written. A rejection sends a new form-error message; the client keeps the form open with the bad fields outlined instead of silently discarding the valid ones.
• The browser (FormLayout, Terminal) stopped self-closing the form on submit — it now waits for the server's verdict (view-terminal on success, form-error on rejection) — and renders <select>s for lookup-bearing fields.

One pre-existing test collision surfaced and was fixed: three tests/Session.test.ts cases used a memory variable named val in a table that also has a column named VAL — under the new precedence rule this now binds to the field. Renamed the variable to m_val throughout (purely mechanical, same asserted outcomes) — verified necessary because the original sequence GETs before any record exists, which would otherwise now throw.

This is the PR that promotes LOOKUP to the full README command reference — both BROWSE and forms now declare, enforce, and render it end to end.

Plan: docs/superpowers/plans/2026-07-11-lookup-columns.md (Tasks 7, 8, 9)

Closes #59.

Test plan

• npm test — 444/444 vitest pass
• npm run build — clean typecheck + build
• npx playwright test — 94/94 pass
• Dedicated coverage in tests/FormFieldBinding.test.ts: field binding + prefill + options; fields shadow memvars; fallback to var when no column matches; hard error with no current record (and no form-open sent); lookup degradation still opens the form; all-or-nothing submit (exact form-error payload, valid sibling field confirmed NOT written); corrected resubmit succeeds; forged form-submit naming an undeclared column can't write it; DATE type validation on a field; grid-exit clears pending state so a stale resubmit is inert; the Test hardening: strict CREATE TABLE parser, demo schema pins, untested WS message types #50 bare-INPUT regression stays fixed