Add unused items pass

[mohammadfawaz]

Adds an UnusedItems warning pass to the compiler. Runs after TypeChecking and emits rustc-style dead_code / unused_variables / unused_imports diagnostics under the UNU code prefix.

What's checked

Item	Warned?
Function (non-entry, non-@test)	✅
Struct (non-record)	✅ (incl. transitive deadness)
const (any scope)	✅
Import	✅
Local let / param / loop iter / const generic	✅
_-prefixed binding that was read	✅
Record	❌ (public surface)
Struct/record field	❌ (too noisy without dataflow + opt-out)
Mapping	❌ (access via intrinsics/storage ops not tracked)
Storage variable	❌ (same)
Interface / function prototype / record prototype	❌

Fields, mappings, and storage variables share one future unlock: data-flow-aware "is this ever read/written" tracking + an @allow_unused attribute for the legitimate false-positive cases.

Leading-_ rules

Mirroring rustc's _x convention. Whether _ is permitted depends on whether the name reaches the Aleo VM — if it does, snarkVM rejects identifiers that do not start with a letter, so we reject at compile time too. Allowed positions silence the corresponding unused_* warning.

Position	Allowed?	Effect / rejection
Local let binding	✅	Silences unused_variable
Tuple-pattern element	✅	Silences unused_variable
Local const	✅	Silences unused_const
Top-level / module const	✅	Silences unused_const
Loop iteration variable	✅	Silences unused_variable
Function parameter (non-entry, non-@test)	✅	Silences unused_variable. Param names don't reach the VM (renamed to r0, r1, …).
Const-generic parameter	✅	Silences unused_variable. Monomorphized away.
Variant::Fn (free fn) name	✅	Silences unused_function. Force-inlined so the name never reaches the VM.
Variant::FinalFn (final fn) name	✅	Silences unused_function. Always inlined.
Interface name	✅	Interfaces are Leo-only — no rejection, no warning.
Entry-point function name (fn inside program { … })	❌	NameValidation rejects → ENV03711002.
Struct / record name	❌	NameValidation rejects → ENV03711002.
Struct / record field name	❌	NameValidation rejects → ENV03711002.
Mapping name	❌	NameValidation rejects → ENV03711002.
Storage variable name	❌	NameValidation rejects → ENV03711002.
Program name (program X.aleo)	❌	NameValidation rejects → ENV03711002.
Binding name matching a bare-callable intrinsic (_self_caller, _block_height, etc.)	❌	Parser rejects → EPAR0370056. The parser dispatches _self_caller() to the intrinsic before any scope lookup, so a same-named local would be silently shadowed.
@no_inline on a _-prefixed Variant::Fn	❌	Type-checker rejects → ETYC0372192. The two annotations directly contradict.

Bonus: reading a _-prefixed local (which would defeat the silencing marker) emits WUNU03714006 — used binding \x` whose name begins with ```.

Architecture

The pass is structured as three phases, all driven by the standard AstVisitor / UnitVisitor traits:

1. UseCollector walks the AST once: populates used_imports / used_globals / potential_global_uses, tracks lexical scopes, emits body-level warnings as each scope drains, and records composite member-dependency edges into a local graph.
2. Pure reachability scan over that dependency graph from "live roots" (records + library top-level structs + any composite directly referenced from user code).
3. UnusedChecker walks the AST again (without descending into bodies) and emits warnings for unused top-level items and imports.

The two visitors share state via an owned CollectedUses struct passed by reference. NameValidation (a sibling pass that runs before TypeChecking) handles all leading-_ rejections in positions where the name reaches the Aleo VM.

[codspeed-hq]

Merging this PR will not alter performance

✅ 17 untouched benchmarks

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_{Comparing mohammadfawaz/unused_items (548b070) with master (2b82c79)}