Surface implicit default bounds in search output

[djdarcy]

Implicit default bounds are not surfaced to users

Problem

When a user runs a search without specifying bounds, the system silently applies hardcoded defaults:

• --max-n: 1,000,000
• --max-m: 10,000

These defaults are asymmetric (100x difference) and invisible in non-verbose mode. A does_exist query returning "Not found" gives no indication that the search was bounded — a user could reasonably interpret this as "no such value exists" when it actually means "not found within [1, 1000000]."

The asymmetry likely reflects the project's origin: n indexes into primesum(n,2) (the primary use case, requiring large ranges), while m is typically used for cross-sequence matching where smaller ranges suffice. But this rationale is not documented.

Current behavior

# User expects exhaustive search, gets bounded search with no indication
python prime-square-sum.py --expr "does_exist primesum(n,2) == 999999999"
# Output: Not found
# (actually: not found in n=[1, 1000000])

# Enumeration silently stops at bounds
python prime-square-sum.py --expr "for_any tri(n) + tri(m)" > results.txt
# Produces 10,000 x 1,000,000 = 10B combinations (m outer, n inner)
# No indication of bounds in output

Desired behavior

• does_exist "Not found" should include the search range, at minimum a hint like (searched n in [1, 1000000])
• for_any completion should show the search space covered
• The --max-n / --max-m asymmetry should be explained in docs
• Consider: always show bounds in non-verbose mode when the user has NOT explicitly set them

Scope

This is about transparency of existing defaults, not about adding unbounded iteration (that's #25) or changing iteration strategies (that's #42).

Affected code

• utils/cli.py — DEFAULT_BOUNDS, build_bounds_from_args()
• prime-square-sum.py — argparse defaults (lines 157-169), verbose output (lines 331-334)
• docs/expressions.md — Variables and Bounds section

Related issues

• Indefinite iteration support (unbounded searches) #25 (parent) — Indefinite iteration support (unbounded searches)
• Multi-variable iteration strategies (product, parallel, adaptive) #42 — Multi-variable iteration strategies
• Multi-level verbosity system (-v, -vv, -vvv) #31 — Multi-level verbosity
• Structured hint and help system for CLI guidance #57 — Structured hint and help system
• Quantifier UX: helpful hints for does_exist + --limit N for bounded for_any enumeration #53 — Quantifier UX and helpful hints