Migrate context accessors to std::ctx / std::prog, reserve Self, enable const generics on final fn

[IGI-111]

Removes the parser-level sugar for execution-context (self.caller, self.signer, self.address, self.id, self.checksum, self.edition, self.program_owner, block.height, block.timestamp, network.id) and for imported-program metadata (Program::checksum, Program::edition, Program::program_owner, Program::function_checksum) in favour of stdlib functions. A new crates/leo-std/src/leo/prog.leo module exposes the program-metadata intrinsics as final fn wrappers parameterised on const generic args (e.g.
std::prog::checksum::[credits.aleo]()).

Each removed form parses as a targeted compiler error pointing at the std::ctx::* or std::prog::* replacement.

Self is promoted to a real lexer keyword so it can be reserved for future use (impl blocks); it errors at every use site.

Three new annotations underpin this migration:

• @offchain propagates an off-chain-only scope restriction from a wrapper to its callers (applied to std::ctx::caller and std::ctx::signer), restoring the access-scope check the parser used to enforce on self.caller / self.signer.
• Compiler-internal @_caller_annotation lets the type checker recognise wrappers that ultimately return _self_caller, so the "caller-as-record-owner" warning fires through arbitrary wrapper chains.
• Compiler-internal @_program_id_arg and @_callable_function_arg fire the program-ID-literal and entry/view-function checks at the user's call site, where the const generic argument is concrete.

Some other required changes and bugfixes:

• std::ctx::id() is now a regular fn (off-chain accessor) rather than a final fn.
• final fns may now declare const generic parameters; they are always inlined into their final {} callsite, so monomorphisation works the same as for regular fns.
• identifier is now an allowed type for const generic parameters (used by std::prog::function_checksum).
• Entry-point fns calling a final fn outside a final {} block are rejected at type-check time, replacing a stale snarkVM diagnostic at deploy time.
• inject_std_library registers every other stub in import_stubs as a parent of std, so dependency programs see std::* as one of their imports.

Migration of in-repo Leo source (tests, examples, doc snippets) and documentation (operator reference, standard-library reference, upgradability and testing guides) is included. New tests cover the removed-syntax errors, Self reservation, std::prog usage, and the now-allowed final fn + const-generic patterns.

There is a companion PR updating the examples here: ProvableHQ/leo-examples#46

[codspeed-hq]

Merging this PR will improve performance by ×2.9

⚡ 8 improved benchmarks
✅ 9 untouched benchmarks

Performance Changes

	Benchmark	BASE	HEAD	Efficiency
⚡	07_type_checking[contrived_settlement]	693.9 ms	75.3 ms	×9.2
⚡	frontend_single_file/control_flow_matrix	997.2 ms	380.3 ms	×2.6
⚡	dependency_chain[01_primitives]	1,023.1 ms	404.6 ms	×2.5
⚡	dependency_chain[02_registry]	1,026.8 ms	414.9 ms	×2.5
⚡	dependency_chain[03_policy]	1,039.3 ms	425.5 ms	×2.4
⚡	dependency_chain[04_router]	1,055.3 ms	439.9 ms	×2.4
⚡	frontend_with_deps/contrived_settlement	1,074.5 ms	456.6 ms	×2.4
⚡	dependency_chain[05_settlement]	1,073.1 ms	458.3 ms	×2.3

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_{Comparing IGI-111/remove_self (30aec8a) with master (65b8b50)}