Add native audio/image efferent surfaces + projector proof-of-shape

audio-surface.el / image-surface.el: own-core additive-synthesis WAV and
raster-PNG renderers (integer-only DSP, since EL has no floats), rendered
from learned engram signatures via a pluggable surface-profile
abstraction (surface-profile.el). audio-demo.el / image-demo.el are
drivers. NOTE: demo files hardcode absolute paths to this worktree's own
directory — will need a path fixup before landing.

elp/projector/ is a Python package the author's own README marks as
"STAGING/PROOF-OF-SHAPE — not the deliverable", superseded by the native
.el surface-profile work above; kept as a validated architecture proof.
Generated output (elp/faculty/{out,sig}, elp/projector/out,
__pycache__) intentionally excluded.

elp/projector/projectors/markdown.py

@@ -0,0 +1,45 @@
+"""markdown.py — the Markdown surface projector (text facet).
+
+The most tractable surface, and the reference implementation: reads the IR's
+realized sentences and lays them out as Markdown. Introduces no content — it is
+pure typography over the faithful text the realizer produced.
+"""
+from __future__ import annotations
+
+import os
+import sys
+
+sys.path.insert(0, os.path.dirname(os.path.dirname(os.path.abspath(__file__))))
+from document_ir import DocumentIR  # noqa: E402
+from projectors.base import register  # noqa: E402
+
+
+class MarkdownProjector:
+    surface = "markdown"
+    media_type = "text/markdown"
+    ext = "md"
+    modality = "text"
+
+    def render_str(self, doc: DocumentIR) -> str:
+        lines: list[str] = [f"# {doc.title}"]
+        if doc.subtitle:
+            lines.append(f"\n*{doc.subtitle}*")
+        abstract = doc.meta.get("abstract")
+        if abstract is not None and abstract.sentences:
+            lines.append("")
+            lines.append(abstract.text())
+        for sec in doc.sections:
+            lines.append("")
+            lines.append(f"{'#' * max(2, sec.level)} {sec.heading}")
+            for block in sec.blocks:
+                body = block.text()
+                if body:
+                    lines.append("")
+                    lines.append(body)
+        return "\n".join(lines) + "\n"
+
+    def project(self, doc: DocumentIR) -> bytes:
+        return self.render_str(doc).encode("utf-8")
+
+
+register(MarkdownProjector())