install.sh: draw a real progress bar during the build steps

[mmelnich]

What this adds

A determinate progress bar during the three long build steps, with three rendering tiers chosen by probing the terminal.

Tier	Condition	Output
2	terminal, colour allowed, UTF-8 locale	bar redrawn in place, ━/─
1	terminal, but NO_COLOR, TERM=dumb, or a non-UTF-8 locale	same bar, #/-
0	not a terminal	one line per step, no escapes, no carriage returns

Tier 0 is a requirement, not a fallback. Redirected output becomes install.log, CI transcripts and bug reports, and control characters make all three unreadable. run_build_step falls straight through to the existing run_step there, so redirected bytes are identical to before this change — and the install-script workflow already asserts that redirected output contains no escape sequence and no carriage return, so this cannot regress silently.

The bar is determinate. Both build tools already report progress on the stream being captured anyway — Ninja writes [12/34], Make writes [ 42%] — so it tracks real work rather than elapsed time. Steps with no parseable progress (clone, configure, verify) keep their existing one-line form rather than growing a bar that means nothing.

Verification

• Redirected: byte-identical to the previous output, and escape-free.
• Under a pty: the bar advances through 5%, 20%, 49% during the RandBLAS build, and every step resolves to a done line.

Notes for reviewers

• The parsing loop necessarily runs in a subshell, so the build command's exit status comes back through a file rather than a variable and is checked explicitly. That is the one non-obvious bit of the implementation.
• The Windows installer does not have this yet; its build steps are short enough that the step counter alone reads fine, and adding it there means dealing with PowerShell 5.1's codepage-437 default, which is a separate problem.
• Stacked on INSTALL.md: document the installers, the tested configurations, and packaging #191 → Add install/install.ps1, and pin the Windows dependency provisioner #190 → Add installer scripts for Linux, macOS and Windows #187 → CMake: state the real version floor, pin the examples fetch, summarize the configuration #186.

[mmelnich]

Folded into #187 at Riley's request ("please combine PRs 187, 190, and 191 — they have no business landing separately"). The commit is unchanged, now sitting on randblas-install-sh alongside the rest of the installer work. Nothing was dropped: the consolidated branch's tree is byte-identical to the state that was tested and green here.