refactor(stage1-r): name the conviction column conviction_at_entry

Rename the dense PositionManagement record's column 9 from `bias_at_entry_abs`
to `conviction_at_entry` (FIELD_NAMES, the `r_col`/test index constants, the
cross-crate layout guard, and the doc comments).

Why: the column holds |bias| at entry, which C10 defines AS conviction (bias =
direction + conviction magnitude; ledger INDEX.md ~458). The record already names
the other half of that decomposition by its domain concept (`direction` = sign,
not `bias_sign`), so naming the magnitude half mechanically (`bias_at_entry_abs`)
was an inconsistency; `conviction_at_entry` speaks the ledger's ubiquitous
language and parallels `direction`, while `_at_entry` preserves the
frozen-snapshot precision.

Considered and rejected: collapsing `(direction, bias_at_entry_abs)` into one
signed `bias_at_entry` and dropping `direction` as redundant — it is not
redundant. On a reversal row col 4 (`direction`) tracks the reopened leg while
col 9 tracks the closed trade's entry conviction, so the two describe different
trades and cannot be losslessly merged; they are also distinct semantic axes
(current-position direction vs per-trade entry conviction).

Scope: domain name at the data boundary, mechanical name internally — the
internal `bias.abs()` variables (`Open.bias_abs`, the fold's `Trade.bias_abs`)
keep their computation-faithful names, exactly the raw-internal / domain-external
split the record uses elsewhere.

Behaviour-preserving: cargo build --workspace clean; cargo test --workspace 500
passed, 0 failed (unchanged); clippy --all-targets -D warnings clean. The layout
guard (`r_col_indices_match_producer_field_layout`) confirms the renamed string.

refs #117

crates/aura-std/src/position_management.rs

@@ -35,7 +35,7 @@ pub const FIELD_NAMES: [&str; WIDTH] = [
     "entry_price",
     "stop_price",
     "exit_price",
-    "bias_at_entry_abs",
+    "conviction_at_entry",
     "size",
     "open",
     "unrealized_r",