docs: reconcile functions roadmap

[nerdalytics]

What changed

• Replace the obsolete conceptual function PR sequence with the current
  implementation status and TinyWhale 0.1 milestone.
• Record the agreed order: explicit call arguments, Never, non-capturing
  function values, indirect lowering, and typed fuzzing.
• Add acceptance behavior for named and inline higher-order function calls.
• Correct historical PR references to feat: add first-class functions (PR 1 of 5) #53, feat: unify expressions and simplify record syntax #54, and feat: everything is an expression (v2) #56.
• Update the list-valued record-field discrepancy to reflect the current
  checker rejection rather than the stale invalid-Wasm behavior.

Why

The roadmap still marked expression unification as pending, linked to a plan
that is no longer present, and described behavior that no longer matches the
compiler. Contributors need a source-grounded continuation point before the
SemIR call-argument refactor begins.

Impact

This is documentation-only. It establishes non-capturing first-class function
values as the next language milestone and identifies the prerequisites and
non-goals without changing compiler behavior.

Validation

• Verified the shipped function history against merged PRs feat: add first-class functions (PR 1 of 5) #53, feat: unify expressions and simplify record syntax #54, and feat: everything is an expression (v2) #56.
• Verified the current call, lambda, None, and list-field behavior against
  compiler source and tests.
• Confirmed stale roadmap links and claims are removed.
• Ran git diff --check successfully.
• Ran the full mise run ci pipeline successfully.
• GitHub Actions Build and Test passed in run 30116369894.