Add try(value, default) special form

[myzie]

Summary

Adds a try(value, default) special form so callers can opt in to graceful failure on a per-expression basis without weakening the strict-missing-key default.

try(int(s), 0)                                      // parse with fallback
try(find(events, it.kind == "purchase").user, "—")  // optional chain
try(user.nickname, nil) || "(none)"                 // present nil

What it traps

• Anything wrapping ErrEvaluate: missing fields/keys, nil selectors, out-of-range indices, type-coercion failures from int / float / etc.

What it does not trap

• Raw context.Canceled and context.DeadlineExceeded — cancellation must remain observable.
• Anything wrapping ErrCompile — defensive; expr should not surface these at Run time, but if it does, that signals a programmer mistake try is not the right place to swallow.

The default expression is lazy: it only evaluates when the primary failed. That means callers can supply expensive or side-effecting fallbacks without paying for them on the success path.

Implementation

• Registered in higherOrderForms and higherOrderNames so dispatch and the did-you-mean suggester pick it up automatically.
• Identifier shadowing (env or WithFunctions) wins over the form, matching every other special form.

Tests and docs

• higher_order_test.go: 12 new tests covering success path, default-on-eval-error, default-on-type-error, lazy default (panicking if invoked), default error propagation, arity, ctx cancellation passthrough, env / func shadowing, nested composition, and try(...) || fallback interop.
• suggest_test.go: tightened to avoid relying on the exact higher-order form count.
• docs/reference/spec.md and llms.txt: new entry plus a paragraph spelling out the trap/no-trap semantics.

Test plan

• go test ./...
• go test -race ./...
• FuzzCompile 20s
• FuzzEval 20s