fix(runtime): mirror surface .0-fallback in ailang_float_to_str

`runtime/str.c::ailang_float_to_str` now applies the `.0`-fallback
already enforced by the surface printer's `write_float_lit`
(crates/ailang-surface/src/print.rs lines 624-637): after the
existing `%g` snprintf, if `x` is finite and the rendered buffer
contains neither `.` nor `e`/`E`, append `.0` so a whole-valued
Float renders with Float-shaped text. The `isfinite(x)` fence
keeps `nan`/`inf`/`-inf` from matching the predicate and turning
into `nan.0`/`inf.0`. The 64-byte stack buffer's truncation guard
is extended by 2 bytes for the fallback path so the defensive
`abort()` discipline holds.

The alternative (document `%g`-without-fallback in the design/
ledger as a deliberate human-friendly contract) was rejected: the
language's stated ethos is machine-readability and round-trip
identity; the surface printer is the reference, runtime drift
from it is a defect not a feature.

Golden updates flow from the contract change. Three fixtures
encoded the old Int-shaped output for Float values:
- `crates/ail/tests/floats_e2e.rs`: `"4\n42\n-1.5\n"` →
  `"4.0\n42.0\n-1.5\n"` (1.5+2.5=4.0, int_to_float(42)=42.0).
- `crates/ail/tests/e2e.rs::mut_sum_floats_prints_55`: `"55"` →
  `"55.0"`. The accompanying doc comment used to canonicalise the
  bug ("`%g` strips the trailing `.0` — so the canonical stdout
  for Float 55.0 is `55`"); rewritten to reflect the new contract.
- `examples/mut_sum_floats.ail` docstring: same correction.

Two doc comments are updated where the post-fix contract differs
from the old text but the fixture golden does not change:
- `examples/float_to_str_smoke.ail` docstring (3.5 still renders
  as `3.5` because `.` is already present — the fallback does not
  fire).
- `crates/ail/tests/e2e.rs::float_to_str_smoke` doc comment (same
  observation, made explicit so the trip-wire's intent is clear).

Verified: `cargo test -p ail --test print_float_whole_e2e` green
(was RED at f19b0dd); `cargo test --workspace` 0 failed across
all crates and integration suites.

closes #7

crates/ail/tests/e2e.rs

@@ -2702,7 +2702,12 @@ fn int_to_str_smoke() {
 /// default applies and the dev target's glibc produces the three
 /// bytes `3.5` (no trailing zeros). If a future target's libc or
 /// locale produces a different rendering, this assertion is the
-/// trip-wire that surfaces it.
+/// trip-wire that surfaces it. Note: post-Gitea-#7 the runtime
+/// also applies a `.0`-fallback when `%g`'s output for a finite
+/// double contains neither `.` nor `e`/`E` (mirroring the surface
+/// printer's `write_float_lit`); `3.5` already contains `.`, so
+/// the fallback does not fire here and the golden stays at the
+/// three bytes `3.5`.
 #[test]
 fn float_to_str_smoke() {
     let out = build_and_run("float_to_str_smoke.ail");
@@ -2866,12 +2871,13 @@ fn mut_counter_prints_55() {
 
 /// Float twin of `mut_counter_prints_55`: `sum_helper` returns the
 /// sum 1.0+...+10.0 = 55.0. The polymorphic `print` routes through
-/// `Show Float.show` → `float_to_str`'s libc `%g` formatter, which
-/// strips the trailing `.0` — so the canonical stdout for Float
-/// 55.0 is `"55"` (matches `examples/floats.ail`'s output of
-/// `4` for `1.5 + 2.5`).
+/// `Show Float.show` → `float_to_str`'s libc `%g` formatter plus
+/// the runtime `.0`-fallback for finite whole-valued doubles whose
+/// `%g` output lacks `.`/`e`/`E` (Gitea #7), so the canonical
+/// stdout for Float 55.0 is `"55.0"` (matches `examples/floats.ail`'s
+/// output of `4.0` for `1.5 + 2.5`).
 #[test]
 fn mut_sum_floats_prints_55() {
     let stdout = build_and_run("mut_sum_floats.ail");
-    assert_eq!(stdout.trim(), "55", "mut_sum_floats must print 55, got {stdout:?}");
+    assert_eq!(stdout.trim(), "55.0", "mut_sum_floats must print 55.0, got {stdout:?}");
 }

crates/ail/tests/floats_e2e.rs

@@ -2,11 +2,12 @@
 //!
 //! `examples/floats.ail.json` is built via the public `ail build`
 //! CLI and run; stdout is matched against the expected three
-//! lines (`4`, `42`, `-1.5`). This test exercises the full
+//! lines (`4.0`, `42.0`, `-1.5`). This test exercises the full
 //! pipeline — Float literal lowering, polymorphic `+` Float arm,
 //! `int_to_float` (sitofp), `neg` Float (fneg), and the polymorphic
 //! `print` for Float (routes through `Show Float.show` →
-//! `float_to_str`, libc `%g`).
+//! `float_to_str`, libc `%g` plus the runtime `.0`-fallback for
+//! whole-valued Float results, per Gitea #7).
 
 use std::path::PathBuf;
 use std::process::Command;
@@ -59,7 +60,7 @@ fn floats_example_prints_expected_stdout() {
     );
 
     let stdout = String::from_utf8_lossy(&run.stdout);
-    let expected = "4\n42\n-1.5\n";
+    let expected = "4.0\n42.0\n-1.5\n";
     assert_eq!(
         stdout, expected,
         "stdout mismatch.\nGot:\n{}\nExpected:\n{}",