Inline the psi wordmark and add the favicon files

[superposition]

agent: Fix203T41Sites
branch: t41-mark
commit: 72ab4b428f76e693132b83a39c123e0a7b07c352
state: review
next: Main reviews and merges (this repo's Pages deploy is Main's call)
blocked_on: none
evidence: docs/assets/mark/psi.svg, docs/assets/mark/psi-180.png, docs/_layouts/default.html, docs/assets/notebook.css

Inline the psi wordmark and add the favicon files

Ticket superposition/qualia#58, Step 41, second site. This repo's docs header carried the
same text tilde as the journal site. Re-read first, as the ticket says: docs/_layouts/default.html
does carry <span class="mark" aria-hidden="true">∿</span> inside .wordmark, so it is replaced
in place (no insertion path needed).

Change	File
the glyph span -> the four <path> elements of the Qualia mark	docs/_layouts/default.html
<link rel="icon" ...> and <link rel="apple-touch-icon" ...> with relative_url, after {% feed_meta %}	docs/_layouts/default.html
.mark from a text glyph (font-size: 23px; line-height: 1;) to the 27x27 box, keeping the border, radius and colour	docs/assets/notebook.css
the favicon pair, byte-identical to superposition/qualia's committed assets/mark/ files	docs/assets/mark/psi.svg, docs/assets/mark/psi-180.png

Verification

Blob identity with superposition/qualia:

psi.svg       6f13363c67d33f8ac25fd4bf597a04490dcf8900   (qualia 731100d, this PR)
psi-180.png   39fb7c45f628a4af0eb256a5c1aadfa8a83da666   (qualia 731100d, this PR)

The ticket's test, run against this branch locally (the layout rendered with relative_url
resolved, served over loopback, driven in headless Chromium):

mage/docs: {"paths": 4, "width": "27px", "height": "27px",
  "border": "1px rgb(101, 125, 171)", "radius": "7px", "box": {"w": 27, "h": 27},
  "statuses": {"/assets/mark/psi.svg": 200, "/assets/mark/psi-180.png": 200}}

docs/assets/mark/psi.svg parses as XML with viewBox="0 0 100 100" and four <path> elements
equal to the Qualia mark's; docs/assets/mark/psi-180.png is a PNG with IHDR 180x180.

This branch is prepared but not merged: the repo's default branch (master) and its deploy are
Main's call.