perf: round 16 — shares-lane & contextMap incremental builders, folder/href/disposition/preview alloc cuts

Finishes the route-level half of the O(N²/page) grouped-listing class ROUND15
fixed inside ResourceList, plus a backend CPU/alloc micro-pack. Every change is
benchmark-gated with a hard rollback rule; no PostgreSQL needed for any arm
(benches/ROUND16.md).

Frontend (vitest):
- F1 "My shares" lanes: the `lanes` $derived.by re-bucketed the whole
  accumulated grant list on every page and every grant edit. SharedLanesBuilder
  re-emits only the fresh page (fan-out + first-appearance header), reusing
  untouched lanes' array refs. 25.5x fewer emit calls, 8.8x wall, O(N²/page)→O(N).
- F2 contextMap (trash/recent/favorites/shared-with-me): each rebuilt a fresh
  N-key Map, re-hashing every accumulated id, per page. primeContextPage holds a
  persistent SvelteMap primed per page (the shipped favoriteIds shape).
  25.5x fewer entry calls, 7.2x wall.
- Extracted the shared O(1) append test (isAppendExtension); F1's gate re-covers it.

Backend (counting-allocator):
- M1 folder display constants Arc::from -> intern_display (3 sites): 3 -> 0 allocs/row.
- M2 build_content_disposition (every download + Range seek): 3 -> 1 alloc, 6x, 2.67x wall.
- M3 nc_href (every NC PROPFIND/REPORT href): Vec+join+format -> one pre-sized
  buffer, keeping urlencoding::encode (byte-identical). 38 -> 27 allocs/op.
- M4 NC preview fileId: collect-then-parse -> borrow-slice parse. 4 -> 0 allocs.

Gates: sharedLanes/listContext.bench.test.ts, examples/bench_round16_micro.rs
(GATE PASS all sections). Frontend: vitest 331 pass, svelte-check clean.
Backend: clippy -D warnings clean, 524 lib tests pass.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0193NjactJVqfU32gxeJDj8m

src/interfaces/nextcloud/preview_handler.rs

@@ -43,12 +43,13 @@ pub async fn handle_preview(
 ) -> impl IntoResponse {
     // Parse the Nextcloud file ID — the NC app may append an instance suffix
     // (e.g. "00000326ocnca"), so strip non-digit characters first.
-    let numeric_part: String = params
+    let digit_end = params
         .file_id
-        .chars()
-        .take_while(|c| c.is_ascii_digit())
-        .collect();
-    let nc_file_id: i64 = match numeric_part.parse() {
+        .as_bytes()
+        .iter()
+        .position(|b| !b.is_ascii_digit())
+        .unwrap_or(params.file_id.len());
+    let nc_file_id: i64 = match params.file_id[..digit_end].parse() {
         Ok(id) => id,
         Err(_) => {
             return Response::builder()

src/interfaces/nextcloud/report_handler.rs

@@ -10,7 +10,7 @@ use quick_xml::{
 use std::collections::{HashMap, HashSet};
 use std::sync::Arc;
 
-use crate::application::dtos::display_helpers::format_file_size;
+use crate::application::dtos::display_helpers::{format_file_size, intern_display};
 use crate::application::dtos::file_dto::FileDto;
 use crate::application::dtos::folder_dto::FolderDto;
 use crate::application::dtos::search_dto::SearchCriteriaDto;
@@ -443,9 +443,9 @@ fn folder_dto_from_search(
         created_at: sr.created_at,
         modified_at: sr.modified_at,
         is_root: sr.is_root,
-        icon_class: Arc::from("fas fa-folder"),
-        icon_special_class: Arc::from("folder-icon"),
-        category: Arc::from("Folder"),
+        icon_class: intern_display("fas fa-folder"),
+        icon_special_class: intern_display("folder-icon"),
+        category: intern_display("Folder"),
         // §14 provenance not selected by search results.
         created_by: None,
         updated_by: None,

src/interfaces/nextcloud/webdav_handler.rs

@@ -186,19 +186,27 @@ pub fn nc_collection_href(username: &str, subpath: &str) -> String {
 pub fn nc_href(username: &str, subpath: &str) -> String {
     let subpath = subpath.trim_matches('/');
     let encoded_user = urlencoding::encode(username);
-    if subpath.is_empty() {
-        format!("/remote.php/dav/files/{}/", encoded_user)
-    } else {
-        let encoded_segments: Vec<_> = subpath
-            .split('/')
-            .map(|seg| urlencoding::encode(seg))
-            .collect();
-        format!(
-            "/remote.php/dav/files/{}/{}",
-            encoded_user,
-            encoded_segments.join("/")
-        )
+    // Write the prefix, user and each encoded segment straight into one
+    // pre-sized buffer — avoids the per-segment `Vec<Cow>`, the joined String and
+    // the `format!` result the previous `.map(...).collect().join("/")` allocated
+    // on every NC PROPFIND/REPORT href (mirrors the native `encode_uri_path`).
+    // Keeps `urlencoding::encode` so the emitted bytes are unchanged.
+    const PREFIX: &str = "/remote.php/dav/files/";
+    let mut out = String::with_capacity(PREFIX.len() + encoded_user.len() + subpath.len() + 8);
+    out.push_str(PREFIX);
+    out.push_str(&encoded_user);
+    out.push('/');
+    // No empty-segment filter: `split('/')` on an empty (root) subpath yields a
+    // single "" whose encode is "" — leaving the trailing slash above intact —
+    // and any internal "//" is preserved byte-for-byte, exactly as the old
+    // `split → map → join("/")` produced.
+    for (i, seg) in subpath.split('/').enumerate() {
+        if i > 0 {
+            out.push('/');
+        }
+        out.push_str(&urlencoding::encode(seg));
     }
+    out
 }
 
 /// Dispatch Nextcloud WebDAV request to the appropriate handler.