web(render): dissolve web::render onto co-located homes (eliminate the projector grab-bag)

[mdorman]

Part of #303 (umbrella); tracked by the #312 shared-UI omnibus (the web::render-eliminated half).

Context

web/src/render/mod.rs is a horizontal grab-bag of ~11 pure server-projector helpers
consumed across ~7 verticals (posts, home, audiences, media, taglist, topbar, avatar, icon).
#312 closes only when web::render is eliminated — its contents redistributed to
co-located feature homes, a shared leaf-primitive home, and the shell home. No open issue
currently owns this; it is the unowned tail of #312.

Scope — redistribute each helper to its co-located home, then delete web::render

Move each helper to live beside its reactive twin / primary consumer (ADR-0070 / #312's
unit-migration rule), a strangler re-export shim per increment:

• Shared leaf primitives — escape_html, Icons, TagCtx → a shared leaf home
  (consumed by avatar/topbar/taglist/posts/audiences markup).
• Home / masthead — render_hero, render_home_masthead → the home vertical.
• Timeline — render_load_more → the timeline/posts render slice.
• Media — format_bytes → the media vertical.
• App-root shell — render_shell, render_head, DEFAULT_THEME are the pure twin of
  App / AppShell; per web: dissolve pages/ui.rs + web::render onto co-located vertical slices (shared-UI omnibus) #312's "shell decided last / migrates as one unit," these move
  with the shell under web: delete pages/ module + target_arch gate; move App/Router to app entry; simplify wasm-clippy #330 (move App/Router to app entry). This issue coordinates with
  web: delete pages/ module + target_arch gate; move App/Router to app entry; simplify wasm-clippy #330 rather than owning the shell.
• Then delete web/src/render/.

Exact destination homes are maintainer-directed (as with the vertical issues); the mapping
above is the starting map.

Acceptance floor

• web::render eliminated; each helper co-located with its twin/consumers (or in the shell
  home via web: delete pages/ module + target_arch gate; move App/Router to app entry; simplify wasm-clippy #330); no fake host stub (ADR-0055).
• cargo xtask validate green at each increment and at close.

Notes

• Not on the web: endgame ratchet — drop js-sys/wasm-bindgen; target_arch only at mod declarations (new xtask check); retire client_only + the #[component] exemption #520 / milestone-14 critical path. web::render is pure host projector
  code and does not affect the wasm target_arch gate that web: endgame ratchet — drop js-sys/wasm-bindgen; target_arch only at mod declarations (new xtask check); retire client_only + the #[component] exemption #520 enforces; web: delete pages/ module + target_arch gate; move App/Router to app entry; simplify wasm-clippy #330's pages/
  deletion also does not require web::render gone (they are separate trees). This closes
  web: dissolve pages/ui.rs + web::render onto co-located vertical slices (shared-UI omnibus) #312's second half but is independent of the web: delete pages/ module + target_arch gate; move App/Router to app entry; simplify wasm-clippy #330 → web: endgame ratchet — drop js-sys/wasm-bindgen; target_arch only at mod declarations (new xtask check); retire client_only + the #[component] exemption #520 chain.

Coordinates with #312 and #330 (shell).