Ship the document root as a UFS disk image, uploaded with IND$FILE

[mgrossmann]

Fixes #252.

static/index.html was in the repository and in no release artifact: the
package installed a server whose DOCROOT had nothing behind it.

The transport, and why it is neither SMP nor XMIT

• SMP has no element type for a DSORG=PS/RECFM=U image, and site content is
  what an APPLY must never touch.
• TSO RECEIVE allocates its own target and refuses to merge into an existing
  dataset -- a first-install-only transport for something an operator replaces.
  (xmit370 could not represent the format either: directory -> PDS,
  --recfm fb|f, LRECL 80.)

So the image ships as a plain file in the archive and goes up over IND$FILE,
which needs nothing on the target beyond a 3270 session -- no mvsMF, no ftpd.

What is in the change

make webroot	builds build/webroot/httpd-webroot.img from static/ with a pinned ufsd-utils, fetched into .mbt/tools/ when it is not on PATH. 1 MB = 256 blocks; --owner/--group are set so a release artifact carries no build-machine userid. package and dist depend on it. Under build/, not dist/: the release workflow publishes dist/* as Release assets, and the image belongs in the archive rather than beside it under a version-less name
project.toml	[distribution] extra puts the image in the zip/tar.gz -- verified, the archive is 6 files now
samplib/httpwebr	allocates HTTPD.@VRM@.WEBROOT.UFS: SPACE=(4096,256), RECFM=U BLKSIZE=4096, primary extent only. Substitution verified in the packed XMIT (HTTPD.V4R0M0D.WEBROOT.UFS)
docs/installation.md	new section 9 -- allocate, upload, mount, verify, replace. Sections 9-11 renumbered to 10-12, anchors and step references updated
smp-todo.md	O3 rewritten: U1-U6 replaced by the decided path, only the SHA256 item left

Measured, not assumed

• Blocking: RECFM=U buffers to BLKSIZE through __fputc (libc370
  @@fputc.c:24), so BLKSIZE(4096) writes whole 4096-byte blocks. The 1 MB
  image is 256 of them, no remainder.
• Options: binary is IND$FILE's default; RECFM then defaults to U
  (ind_file370 indparse.c:249).
• Pre-allocation: IND$FILE allocates an existing dataset DISP=SHR and takes
  its DCB (indmain.c:190, :273). That is why the documented upload carries
  no options at all -- and why it does not depend on which IND$FILE build a
  system has.
• The trap: the same DISP=SHR rewrites a dataset UFSD has mounted.
  UNMOUNT PATH=/www is a numbered step in the doc, not a footnote.
• Detection: a BLKSIZE mismatch surfaces at MOUNT as UFSD062E SUPERBLOCK VALIDATION FAILED (ufsd ufsd#sbl.c:69), not as a corrupt page.
• / serves it: httpget.c:68 appends index.html to a directory request.

Encoding: nothing to configure, and no HTTPPRM0 change

http_send_file() translates UFS files with the hard-coded IBM-1047 table,
independent of CODEPAGE= (src/httpfile.c:70) -- exactly what ufsd-utils
writes. Measured end to end: ufsd-utils cp -> stored bytes -> ibm1047_etoa
returns static/index.html byte for byte, UTF-8 sequences included. Over all
256 byte values only 0x85 and 0xF7 fail to round-trip; the Makefile says so.

CODEPAGE=IBM1047 in the shipped HTTPPRM0 was considered and rejected on
measurement. cc370 compiles literals in CP037 (common/src/mvs370.c), and
CODEPAGE= governs those: under IBM1047 a [ compiled as 0xBA leaves as
0xDD, which would break every mvsMF array response -- the primary route in the
shipped configuration -- plus HTTPD's own login form and error pages. Checked
against a live server: [ currently arrives as 0x5B.

Verification

• make package -- clean, 6 files in the archive, image included
• make dist -- re-renders with the image present
• image inspected with ufsd-utils ls -l / info: 256 blocks, IBMUSER/SYSPROG,
  index.html 3573 bytes
• samplib XMIT listed and extracted: HTTPWEBR present, @VRM@ substituted
• ufsd-utils bootstrap exercised from an empty .mbt/tools/

Not verified on MVS: the upload itself. IND$FILE is present on the reference
system (SYS2.CMDLIB), but a 3270 transfer is interactive -- every claim about
its behaviour above comes from the ind_file370 sources.