docs: fix the four front-door examples that misrepresented the language

[InauguralPhysicist]

The first artifacts a stranger reads — the README and the intro docs — were wrong on the exact surfaces we lead with. Every corrected example was verified against the real runtime.

What was broken

• README observer example (README oscillating example prints diverging — the sequence is too short to ever classify as oscillating #313) — 5,2,5,2 → report → "oscillating" actually printed diverging (four samples is too short to classify). This is the showcase of the observer, our whole differentiator, printing the wrong trajectory. Extended to three periods so it truly prints oscillating.
• README stdlib snippet (README stdlib snippet uses an undefined double and errors out (undefined variable 'double') #319) — map of [[1,2,3], double] referenced an undefined double and crashed with undefined variable 'double' on a reader's first copy-paste. Now defines double inline → [2, 4, 6].
• lib/format.eigs fmt_bar docstring (lib/format.eigs docstring shows wrong fmt_bar output (████░░░░░░ vs actual [######....] 60.0%) #320) — showed ████░░░░░░; actual output is [######....] 60.0%. Docstring now matches.
• String-escape docs (String escape docs are out of sync with the lexer: \r is supported but undocumented; \{/\} are documented but only work via the catch-all branch #318) — SYNTAX/GRAMMAR/LANGUAGE_CONTRACT implied a closed set \n \t \r \\ \" \{ \}. The lexer recognizes \n \t \r \\ \" and drops the backslash on any other \x (so \{/\} yield literal braces via the catch-all; \r was real but under-documented). Docs now state the true contract.

Verification

All corrected examples run byte-true against the current runtime:

• README oscillating example prints diverging — the sequence is too short to ever classify as oscillating #313 → oscillating
• README stdlib snippet uses an undefined double and errors out (undefined variable 'double') #319 → [2, 4, 6]
• lib/format.eigs docstring shows wrong fmt_bar output (████░░░░░░ vs actual [######....] 60.0%) #320 → [######....] 60.0%
• String escape docs are out of sync with the lexer: \r is supported but undocumented; \{/\} are documented but only work via the catch-all branch #318 → \r→CR, \{/\}→{/}, \q→q (catch-all confirmed)

Docs-only; tools/doc_drift_check.sh passes.

Closes #313
Closes #319
Closes #320
Closes #318

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