Add doc-test workflow for introduction.md examples

[keiravillekode]

Summary

• New script `bin/test-doc-examples` extracts every ```factor``` block from `concepts//introduction.md` and `exercises/concept//.docs/introduction.md`, runs each file's blocks back-to-back inside the `exercism/factor-test-runner` image with a single `docker run`, and checks that each `! => ...` line appears as a subsequence of Factor's actual stdout.
• New workflow `.github/workflows/doc-test.yml` wires it into CI on the same shape as `test.yml` (`ubuntu-24.04`, pull the image, run the script).
• Fixes the 26 doc bugs the script surfaced on `main` so the workflow lands green. Final score: 149/149 blocks pass across 48 files.

Why

The kind of bug we just fixed in #190 (! => 16/3 where Factor actually prints 5+1/3) is exactly what this catches automatically. Initial baseline against main found 26 such bugs across 14 files — wrong printed outputs, undefined-word typos, swapped `change-nth` args, etc.

Notable doc fixes

• `concepts/tuples`: `point new` actually prints `T{ point }`, not `T{ point { x 0 } { y 0 } }` (Factor pprint omits slots at their initial values).
• `concepts/strings`: `.s` output was rendered as `"foo" / "bar"` on one line, but Factor prints stack items one per line; the `surround` example had 4 strings and a TODO note (`"... err, surround takes 3"`).
• `concepts/quotations` & `high-school-sweetheart`: `{ 1 [ 2 + ] [ 3 * ] } concat` doesn't type-check — `concat` needs sequence elements.
• `pursers-pantry`: hashtable iteration order isn't tied to insertion order; drop the brittle assertion. Block 2 had `set-at` args swapped (printed `H{ { 5 "coal" } }` instead of `H{ { "coal" 5 } }`).
• `mosaic-mischief`: `change-nth` args were `[ 1 + ] 0 pick` which mangles the stack; reorder to `0 over [ 1 + ]`.
• `lighthouse-logbook`: blocks referenced an undefined `my-log`; add a `: my-log` definition.
• `secrets`: multi-line parenthetical commentary inside an `! => -12 (... )` comment was being parsed as part of the expected output; lifted into prose.
• `streams` block 1: `stream-contents` needs the stream on the stack, but inside `with-input-stream` it's bound to the dynamic `input-stream` variable; use `read-contents`.
• Marked `! DOCTEST: SKIP` on a few intentionally-untestable blocks (re-definitions for comparison, illustrative `throw` outside a `recover`, `:>` outside `[let`, hypothetical helpers).

Script details

• Each fenced ```factor``` block in a file gets composed into one Factor program, with a sentinel `print` between blocks so we can localise failures.
• `! => result` lines are extracted as expected output. Trailing prose (` (a proper ratio)` etc.) is stripped before comparison.
• Subsequence matching tolerates demo lines that print without an explicit `! =>` — extra actual output is fine, but expected lines must appear in order.
• If no block in a file declares `USING:` we prepend a generous default; otherwise we trust the doc (mixing the default with a per-block `USING:` provokes ambiguous-word errors — `ascii` and `unicode` both define `blank?`/`LETTER?`).
• A block can opt out with `! DOCTEST: SKIP`.

Test plan

• CI on this PR runs `Doc-test` to green.
• CI on this PR continues to run `Track Exercises on Runner` to green.