ANW-26 HTTP /query markdown-merge mode

Accept: text/markdown on /query concatenates matched note bodies into one
markdown document, each fragment preceded by an HTML-comment source marker.
Adds __anw-order (frontmatter-key ordering, asc/desc, missing-key-last) and
__anw-kind (homogeneity guard, 400 on a mixed merge). Both evaluate over the
full matched set before __anw-limit truncates. Merge mode uses its own
body-cloning projection; the JSON path is unchanged (no body clone).

tests/hurl/query/merge_kind_guard_400.hurl

@@ -0,0 +1,8 @@
+# [ANW-26] __anw-kind=status rejects a mixed merge: alpha (draft) and anwesen
+# (active) carry distinct status values, so the guard returns 400.
+GET {{host}}/query?__anw-path=Projects&status__exists=true&__anw-kind=status
+Accept: text/markdown
+HTTP 400
+[Asserts]
+body contains "draft"
+body contains "active"

tests/hurl/query/merge_kind_guard_ok.hurl

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+# [ANW-26] __anw-kind=kind passes when every matched note carries the same
+# value (both PDRs have kind: PDR).
+GET {{host}}/query?__anw-path=Projects&kind=PDR&__anw-kind=kind
+Accept: text/markdown
+HTTP 200
+[Asserts]
+body contains "First PDR."
+body contains "Second PDR."

tests/hurl/query/merge_markdown.hurl

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+# [ANW-26] Accept: text/markdown on /query merges matched note bodies into
+# one document, each fragment preceded by its source marker.
+GET {{host}}/query?__anw-path=Projects&kind=PDR
+Accept: text/markdown
+HTTP 200
+[Asserts]
+header "Content-Type" == "text/markdown; charset=utf-8"
+body contains "<!-- source: Projects/PDR-001-intro.md -->"
+body contains "<!-- source: Projects/PDR-002-followup.md -->"
+body contains "First PDR."
+body contains "Second PDR."

tests/hurl/query/merge_order_desc.hurl

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+# [ANW-26] __anw-order=num:desc orders fragments by the `num` key descending,
+# so PDR-002 (num 2) precedes PDR-001 (num 1) in the merged body.
+GET {{host}}/query?__anw-path=Projects&kind=PDR&__anw-order=num:desc
+Accept: text/markdown
+HTTP 200
+[Asserts]
+body matches /(?s)Second PDR.*First PDR/