LLBC: name inherent methods instead of aborting, and deduplicate the name builders

[feliperodri]

Rebased onto main after #4881 merged; three commits: 29c3b1eb2, 697526953, and dfc1b0592 (see below).

Second step toward the Charon bump (#4834), still on the old pin.

The bug. Calling any inherent method aborts the compiler with the LLBC backend. A local type is enough:

impl Counter { fn get(&self) -> u32 { self.n } }

expected impl trait, found inherent impl on DefId(0:6 ~ test[4a52]::{impl#0})

and so is x.wrapping_add(y) from core. Method names get the implementing type appended so methods from different impls don't collide, and that type was read from the impl's trait ref — which only trait impls have. It now comes from the impl's self type, which both kinds have and which prints identically for trait impls: traitimpl (pinned in #4881) still expects get_valA / get_valB. Two new tests cover the local and the core case; both crashed before.

The cleanup. The path-to-name walk existed three times (defid_to_name, def_to_name, adtdef_to_name), differing only in where the span came from — and the ADT copy panicked on an {impl} path element the other two skip. Now there's one. Separately, the integer-type → scalar-value table was duplicated between constants and switch targets; it's one function now, so the bump rewrites it in one place.

No behaviour change beyond the crash — that's the point of doing this before the bump. All 19 expectations from #4881 are untouched, including int_literals (negatives at every width) and switch_int, which exercise the merged table.

The third commit stops inherent_core/expected from pinning the callee's @FunN id: ids follow the fingerprint order in which Kani reaches items, which changed with nightly-2026-09-24 (@Fun1 became @Fun2). The callee's name is still checked by its declaration line.

Where I'd like input: the suffix scheme itself (getCounter, core::num::wrapping_addu8) is kept as-is, because changing it here would muddy the "nothing changed" signal. Charon's own naming uses a proper {impl} path element instead; I'd rather adopt that in the bump, where names change anyway. Shout if you'd prefer it now. (Now done on top of the bump, in #4886; see #4884.)

I also dropped a planned "route every redesigned construct through helpers" step after looking closely: those call sites already sit in five functions (translate_ty, translate_allocation, translate_switch_targets, translate_bin_op, the trait-decl builder), so a helper layer would add indirection without making the bump meaningfully smaller.

tests/llbc 21/21, kani-llbc-regression.sh, fmt, clippy. Part of the names item in #3585.