parse-reader: splice multi-line spec continuations so -> stays top-level

[kumavis]

EigenTrust pitfall #6 (2026-04-23) reported that a multi-line spec like

spec eigentrust-step
     [List [List Rat]]   ;; matrix C
     [List Rat]          ;; pre-trust p
     Rat                 ;; damping
     [List Rat]          ;; current t
     -> [List Rat]       ;; next t
defn eigentrust-step [c p alpha t] ...

failed with spec: spec type for eigentrust-step has no arrow but defn has 4 params. The user attributed it to line comments, but those are
stripped by the tokenizer; the real cause is indent grouping. Each
continuation line with multiple tokens was wrapped via wrap-stx-list
into a sub-list, so the line -> [List Rat] became (-> (List Rat)).
split-on-arrow-datum only scans the top level of the spec body
tokens, so the buried arrow was invisible and decompose-spec-type
treated the body as a zero-arrow relation type.

For nodes whose first token is spec or spec-, switch
tree-node->stx-elements to a flatten variant that splices indent-
grouped continuation lines directly into the parent token stream.
Metadata-style continuations whose first token is a keyword-like
symbol (:doc, :where, :method, ...) are still wrapped, so the
existing process-spec metadata loop continues to recognize them.

Bracket-grouped function-type parameters like [-> A Bool] are
unaffected — they come from explicit brackets, not indent grouping,
and remain sub-lists with -> as the head. Other forms (defn,
def, match, ...) are unchanged: only spec-form nodes take the new
path.

Test coverage in test-spec-multiline-ws.rkt covers the eigentrust
reproducer, the metadata-continuation case, the bracketed prefix-arrow
function-type case, and a non-regression check on defn body
indent-grouping.

https://claude.ai/code/session_01MbncYJnrvjzhbVWw4xGi5x

[kumavis]

looks good, requested an additional type to ensure it's compatible with additional keywords

[kumavis]

requested test was added 👍

[hierophantos]

LGTM. Surgical scope (one fork in tree-node->stx-elements + 3 local helpers); other forms (defn, def, match) untouched; metadata-keyword exception correctly preserves :doc / :where / :method continuation lines as wrapped sub-lists for process-spec's metadata loop.

Audit-passed against our codebase: 935 existing specs are all single-line, so zero existing code is affected. The fix is purely additive — unblocks multi-line type signatures, which we'll want for complex dependent / session / effect types that are core to the language vision.

Approving.