feat: apply per-canister moc args in check and check-stable

[Kamirus]

Problem

[canisters.<name>].args from mops.toml were only passed to moc during mops build. The check and check-stable commands silently ignored per-canister args, using only global [moc].args.

This meant flags like --enhanced-migration=migrations/backend worked in mops build but were missing during mops check, causing check failures. The only workaround was setting the flag in global [moc].args, which doesn't work for multi-canister projects where each canister needs a different migration path.

Fix

Per-canister [canisters.<name>].args are now applied in all three commands (build, check, check-stable) with consistent ordering: [moc].args → [canisters.<name>].args → CLI -- flags.

New CLI capabilities

mops check now accepts canister names:

mops check              # check all canisters
mops check backend      # check a specific canister
mops check src/main.mo  # check a file (unchanged)

mops check-stable now supports a no-args mode:

mops check-stable              # check all canisters with [check-stable] configured
mops check-stable backend      # check a specific canister
mops check-stable old.most     # check using an old file (unchanged)

Both commands apply [canisters.<name>].args when checking canisters, matching mops build behavior.

Docs updated

• mops check — updated arguments, examples, and description for canister-name support
• mops check-stable — documented canister mode and file mode as separate usage patterns
• mops.toml — clarified that [canisters.<name>].args applies to check, check-stable, and build; added multi-canister example

Test plan

• mops check with no args checks all canisters with per-canister args
• mops check backend filters to a specific canister
• mops check nonexistent errors with available canister names
• [canisters.X].args with -Werror correctly applied in check
• mops check-stable with no args checks all canisters with [check-stable] config
• mops check-stable backend filters to a specific canister
• [canisters.X].args with --enhanced-migration correctly applied in check-stable
• All 45 existing check/check-stable tests pass