fix(ir): const evaluators' field-access arm is raw value-shadow aware [F0.11]

A backtick raw value-shadow receiver (`` `f64 := … `` then `` `f64.epsilon ``,
`` `s8.max ``) was misclassified as the builtin numeric-limit accessor by the
shared compile-time evaluators. The sibling `isFloatValuedExpr` already guards
this with an `is_raw` check, but `evalConstFloatExpr` / `evalConstIntExpr` did
not — so once a raw value-shadow's field read flowed into the unified float→int
narrowing rule or an array-dim count, the float folder returned the BUILTIN
`f64.epsilon` (2.22e-16) and wrongly errored, and the integer folder turned
`` `s8.max `` into the builtin `127` (a fabricated 127-element array).

Both evaluators' field-access arms now mirror `isFloatValuedExpr`'s `is_raw`
guard: a raw receiver yields `obj_name = null`, so it is never a
numeric-limit/pack leaf and falls through to the ordinary runtime field read. A
raw value-shadow is a mutable-local field (an observable later reassignment),
so it is genuinely runtime and must not be const-folded — it now behaves exactly
like a plainly-named field read: `` `f64.epsilon `` narrowing into `s64`
truncates its field value (11.5 → 11, identical to `b.epsilon`), and `` `s8.max ``
as an array dimension is rejected as a non-constant count (identical to `b.max`).
The bare builtin path is unchanged.

Regression (issue 0095 / F0.11-7):
- examples/0169-types-value-shadow-field-narrowing.sx (positive — raw float-field
  read narrows/truncates, mutation proves runtime, bare limit still folds)
- examples/1148-diagnostics-value-shadow-field-dim-not-const.sx (negative — raw
  int-field dim rejected as non-const)
- program_index.test.zig "a backtick raw-shadow receiver is a field read, not a
  numeric-limit fold (F0.11-7)"

specs.md + readme.md note the value-shadow rule extends into the narrowing/count
contexts.

examples/expected/0169-types-value-shadow-field-narrowing.exit

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

examples/expected/0169-types-value-shadow-field-narrowing.stderr

@@ -0,0 +1 @@
+

examples/expected/0169-types-value-shadow-field-narrowing.stdout

@@ -0,0 +1,3 @@
+x=11 y=11 yb=11
+xm=4
+lim=0

examples/expected/1148-diagnostics-value-shadow-field-dim-not-const.exit

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

examples/expected/1148-diagnostics-value-shadow-field-dim-not-const.stderr

@@ -0,0 +1,5 @@
+error: array dimension must be a compile-time integer constant
+  --> examples/1148-diagnostics-value-shadow-field-dim-not-const.sx:27:13
+   |
+27 |     arr : [`s8.max]f32 = ---;
+   |             ^^^^^^

examples/expected/1148-diagnostics-value-shadow-field-dim-not-const.stdout

@@ -0,0 +1 @@
+