fix(ir): unify float→int narrowing — integral folds, non-integral errors [F0.11]

Issue 0095: a typed local/param/field silently TRUNCATED a float initializer
to an integer annotation (`y : s64 = 1.5` → 1) with no diagnostic. Agra ruled
the UNIFIED rule (Option B): an implicit float→int in a typed binding behaves
like the array-dimension rule —

  - an INTEGRAL compile-time float FOLDS to its int (`4.0` → 4, `-2.0` → -2);
  - a NON-integral float is a COMPILE ERROR (`1.5`, `4.5`);
  - explicit `xx` / `cast(T)` ALWAYS truncates (the escape hatch).

Applied consistently to typed local / param-default / field-default, typed
module CONST, and array dim — all reusing the single
`program_index.floatToIntExact` / `evalConstIntExpr` facility (no second
integral check).

- `Builder.constFloatInfo` reads a compile-time `const_float` back from its
  Ref (value + span).
- `coerceToType` is now the IMPLICIT path: its `.float_to_int` arm folds an
  integral const-float to `constInt`, else emits the narrowing diagnostic.
  `coerceExplicit` is the raw truncating path; `xx` (lowerXX) and `cast(T)`
  route through it so the escape still truncates.
- Field-default lowering (struct-literal pad, named-field default,
  buildDefaultValue) now coerces the default to the field type at the IR level
  (was silently bit-coerced by emitStructInit).
- Const path: `typedConstInitFits` accepts an integral float (literal or a
  `M + 2.0`-style expression folding via `evalComptimeInt`); `emitModuleConst`
  / `constExprValue` / `globalInitValue` fold an integral float to its int and
  reject a non-integral one — relaxing F0.7's blanket float rejection.

Tests: examples/0168 (positive: local/field/param/const fold, xx/cast
truncate), examples/1146 (negative: local/param/field error), integral-float
const cases added to examples/0162; non-integral const cases in 1143 stay
errors. specs.md + readme.md document the unified rule, cross-referencing the
array-dim rule. issues/0095 marked RESOLVED.

examples/expected/0162-types-typed-module-const-roundtrip.stdout

@@ -4,3 +4,4 @@ S=hi
 P_is_null=true
 KE=4 len=4 WE=4.000000
 MF=2.500000 MFR=2.500000
+KF=4 len=4 KFE=4

examples/expected/0168-types-integral-float-to-int.exit

@@ -0,0 +1 @@
+0

examples/expected/0168-types-integral-float-to-int.stderr

@@ -0,0 +1 @@
+

examples/expected/0168-types-integral-float-to-int.stdout

@@ -0,0 +1,6 @@
+local=4
+neg=-2
+field=4
+param=6
+const=8 len=8
+xx=4 cast=1

examples/expected/1146-diagnostics-nonintegral-float-to-int.exit

@@ -0,0 +1 @@
+1

examples/expected/1146-diagnostics-nonintegral-float-to-int.stderr

@@ -0,0 +1,17 @@
+error: cannot implicitly narrow non-integral float '1.5' to 's64'; use an explicit cast (`xx`/`cast`)
+  --> examples/1146-diagnostics-nonintegral-float-to-int.sx:21:15
+   |
+21 |     y : s64 = 1.5;      // non-integral local initializer → error
+   |               ^^^
+
+error: cannot implicitly narrow non-integral float '3.5' to 's64'; use an explicit cast (`xx`/`cast`)
+  --> examples/1146-diagnostics-nonintegral-float-to-int.sx:15:15
+   |
+15 |     f : s64 = 3.5;      // non-integral field default → error
+   |               ^^^
+
+error: cannot implicitly narrow non-integral float '2.5' to 's64'; use an explicit cast (`xx`/`cast`)
+  --> examples/1146-diagnostics-nonintegral-float-to-int.sx:18:26
+   |
+18 | badDefault :: (x : s64 = 2.5) -> s64 { return x; }   // non-integral param default → error
+   |                          ^^^

examples/expected/1146-diagnostics-nonintegral-float-to-int.stdout

@@ -0,0 +1 @@
+